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Vanadium dioxide (VO2) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of VO2 thin films across the phase transition for the first time using variable temperature Kelvin force microscopy (KFM). The work function of VO2 is estimated to be 5.15 eV in the insulating phase and increases by approximately 0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid state devices incorporating complex oxides containing multi-valent cations.