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During the developmental period spanning from middle adolescence (around 15 years old) to young adulthood (up to 25 years old), transitional-age youth (TAY) encounter numerous developmental and social challenges (Wilens & Rosenbaum, 2013). For instance, TAY have to form an identity, navigate romantic and social relationships, and develop into self-reliant, autonomous adults. Additionally, they are required to flexibly adapt to new environments and situations. While many TAY successfully navigate this transition, a significant number of TAY experience this period as stressful and develop mental health problems. Consequently, TAY are more vulnerable to experience psychological problems compared to other age groups (i.e., adolescents under the age of 15 or adults over the age of 25; Whiteford et al., 2013). This is concerning as TAY with psychological problems are more likely to face challenges in their educational- and professional career, and experience a diminished quality of life (Patel et al., 2007). Furthermore, if left untreated, these mental health issues may persist into adulthood (Costello & Maughan, 2015; Gustavson et al., 2018). Considering this, it is crucial to enhance our understanding of how to optimize psychological interventions for this specific age group. One approach to optimize psychological interventions for TAY is to investigate the mechanisms through which the interventions induce change. As Kadzin et al. (2007) argue, examining mechanisms of change in interventions is relevant for several reasons. First, there are numerous interventions that might works through similar underlying mechanisms. Identifying a few key mechanisms of change might bring order and parsimony to the field of developmental psychology. Second, if we know how change comes about and what the key mechanism of change in interventions are, we can develop better strategies to improve these mechanisms. Third, identifying mechanisms of change can optimize the generalizability of intervention effects from research to clinical practice. While randomized controlled trials inform clinicians about which evidence-based intervention protocols to use, studies investigating mechanisms of change provide insights into the critical ingredients of interventions that should be preserved in clinical practice. Last, investigating mechanisms of change in interventions also provide insights into the development and maintenance of psychological problems in TAY, beyond the therapeutic context. This knowledge contributes to a broader understanding of risk and protective factors for psychological problems in this age group. To improve the effectiveness of current psychological interventions, clinicians and researchers have developed innovative treatments that complement to traditional therapeutic paradigms. An example of such an intervention is Acceptance and Commitment Therapy (ACT), a third wave cognitive behavioral therapy (CBT) developed at the end of the 20th century (Hayes et al., 2004). The main goal of ACT is to increase psychological flexibility: “the ability to contact the present moment more fully as a conscious human being, and to change or persist in behavior when doing so serves valued ends”(Hayes et al., 2006, p.7). According to the ACT theory, a lack of psychological flexibility is considered to be the underlying mechanism of different types of psychological problems and of reduced quality of life (Hayes et al., 2006). There are meta-analyses showing that ACT is equally effective compared to well established evidence-based treatments (e.g. CBT) and superior to inactive control conditions (e.g., placebo or waitlist) and treatment as usual (TAU) in reducing different types of psychological problems in adults (Gloster et al., 2020), children and adolescents (Fang en Ding., 2020) and TAY (Keulen et al., under review). Notwithstanding, less is known about the mechanisms through which ACT works. Hence, the goal of the current study was to examine four possible mechanisms of change in acceptance and commitment therapy for TAY. A first potential mechanism of change in ACT is psychological flexibility, the mechanism through which, according to the ACT theory, ACT enables change. Psychological flexibility consists of six core processes: acceptance (i.e., learning to accept difficult emotions, thoughts, and situations rather than avoiding or resisting them), cognitive defusion (i.e., altering the impact of thoughts rather than their form, frequency, or intensity), present-moment awareness (i.e., focusing on the here and now instead of ruminating about the past or worrying about the future), self-as-context (i.e., cultivating a flexible sense of self where content about the self can be observed and accepted), values (i.e., identifying what truly matters), and committed action (i.e., aligning behavior with one's values; Hayes et al., 2006). In ACT, these processes are fostered through psychoeducation, mindfulness, metaphors, and experiential exercises. There are some review studies supporting the idea that psychological flexibility is a mechanism of change in ACT (Hayes et al., 2022; Murillo et al., 2022; Ren et al., 2019; Stockton et al., 2019). However, the methodological quality of the available mediation literature remains low (e.g., mediators and outcomes are often measured at only two timepoints). A second possible mechanism of change in ACT is self-compassion. Self-compassion comprises three key elements: treating oneself with kindness, recognizing one's shared humanity, and being mindful when considering negative aspects of oneself (Neff et al., 2003). Many ACT interventions, including the one examined in this study, incorporate exercises designed to enhance all three key elements of self-compassion. Some studies also support the notion that self-compassion is as a mechanism of change in ACT. For example, self-compassion has been shown to mediate the effects of ACT on concerns over mistakes (Ong et al., 2019) and depressive symptoms (Ritzert et al., 2020). Nonetheless, research on the mediating role of self-compassion in ACT remains scare. A third mechanism of change within ACT might be adaptive emotion regulation. The adaptive coping with emotions (ACE) model of Berking and colleagues (2014) outlines nine key skills that are essential for adaptive emotion regulation: attention to emotions, clarity of emotions, body awareness, understanding the cause of emotions, modifying emotions, acceptance of emotions, tolerating emotional distress, engaging in goal-directed behavior and self-support in emotionally distressing situations. Some of these skills are directly targeted in ACT. For example, through mindfulness, TAY may become more aware of their emotions and bodily sensations; the acceptance and defusion components in ACT could help TAY accept unwanted emotions and increase their tolerance for emotional distress; committed action could encourage TAY to act in line with their values and engage in goal-directed behavior; and the self-compassion component in ACT might foster self-support in TAY. Some studies indeed show that ACT can increase clients’ emotion regulation, which in turn may reduce their borderline personality symptoms and feelings of hopelessness (Morton et al., 2012) and aggressive behavior (Zarling et al., 2015). Nevertheless, these studies were solely conducted in adults samples and therefore less generalizable to TAY. We propose autonomy as a fourth mechanism of change in ACT. Autonomy can be defined as one’s ability to choose and define goals, feel confident about one’s own choices and goals and develop a strategy to achieve one’s goals (Noom et al., 2001). Individuals with lower levels of autonomy are more vulnerable to experience psychological problems than individuals with higher levels autonomy (Ryan et al., 2016). To our knowledge, there are no studies investigating if the effects of ACT are mediated by the clients’ autonomy. However, we expect that autonomy could be an important mechanism of change in ACT. Namely, ACT encourages TAY to make more self-directed choices and act in alignment with their values, regardless of internal of external barriers. Moreover, the ACT intervention that is investigated in this study is explicitly designed for TAY and puts specific emphasis on fostering autonomy. Hence, there are some indications that psychological flexibility, self-compassion, adaptive emotion regulation and autonomy are important mechanisms of change in ACT, but more research is needed. Especially in younger samples, as most previous mediation studies are performed in adult samples. Additionally, the quality of ACT mediation studies needs improvement. For example, many studies measured mediators and outcomes at only two timepoints, which does not meet the temporal precedence requirement for mediation analysis. Furthermore, not all studies compared mediation effects across different interventions, making it challenging to identify intervention-specific mediators (Maric et al., 2012). Therefore, the goal of the current study is to examine whether psychological flexibility, self-compassion, adaptive emotion regulation, and autonomy serve as intervention-specific mechanisms through which ACT effectively reduces psychological problems. Specifically, we will first examine if and how TAY’s levels of psychological flexibility, self-compassion, adaptive emotion regulation, autonomy and psychological problems change during the intervention and if these changes are different in the ACT compared to the TAU condition. We hypothesize that in both conditions psychological flexibility, self-compassion, adaptive emotion regulation and autonomy will increase and psychological problems will decrease. However, based on earlier meta-analyses (Gloster et al., 2020; Keulen et al., under review) we expect the increases in in psychological flexibility, self-compassion, adaptive emotion regulation, autonomy and the decreases in psychological problems to be steeper in the ACT condition than in the TAU condition. Second, we will test our hypothesis that increases in psychological flexibility, self-compassion, adaptive emotion regulation, and autonomy are associated with concurrent decreases in psychological problems in both conditions. Third, we will examine whether changes in psychological flexibility, self-compassion, adaptive emotion regulation and autonomy serve as a potential mediating link between ACT and psychological problems. Particularly, we hypothesize that ACT will enhance psychological flexibility, self-compassion, adaptive emotion regulation and autonomy, and that increases in these mechanisms subsequently reduce psychological problems in TAY. Although this model, given the theoretical foundation and focus of ACT, is the most plausible, we will also test an alternative model suggesting that changes in psychological problems precede changes in psychological flexibility, self-compassion, adaptive emotion regulation, and autonomy. To answer these research questions, we will use data from a randomized controlled trial comparing ACT with treatment as usual (TAU; Keulen et al., 2023). We will apply parallel process latent growth curve modeling (LGM; Cheong et al., 2003) to analyze the data, which offers several advantages over more traditional regression-based methods. Particularly, LGM uses multiple data points , providing more detailed insights into changes over time and increasing the reliability of assessing change (Willett, 1989). Also, LGM estimates individual differences in change over time, making it more representative of real-life situations where trajectories of change vary significantly among TAY (Cheong et al., 2003). Furthermore, LGM estimates change in the mechanism (i.e., psychological flexibility, self-compassion, adaptive emotion regulation, and autonomy ) and change in the outcome (i.e., psychological problems) simultaneously. This allows changes in the mechanism to be linked to changes in the outcome, enhancing mediation analysis in longitudinal studies. Finally, LGM has significantly more power to detect intervention effects compared to more traditional regression-based methods (Stull et al., 2008).