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The effects of impurities - hydrogen, water vapour and sodium chloride - on shock wave stability and structure in ionizing-argon and krypton flows were investigated by using a dual-wavelength Mach-Zehnder interferometer in conjunction with the UTIAS 10cm x 18cm (4in x 7in) Hypervelocity Shock Tube. At shock Mach number of about 15, when pure argon or krypton is used as a test gas, the resulting translational shock front develops sinusoidal oscillations. The subsequent ionization relaxation region, the electron-cascade front and the quasi-equilibrium state also exhibit unstable characteristics. The addition of small amounts of hydrogen ( or = 0.5% by pressure) to the test gas stabilizes the entire flow and at the same time drastically shortens the relaxation length to about 1/3 of the original value in argon and 2/3 in krypton. The addition of about 1% of water vapour (by pressure) to the argon test gas also stabilizes the flow and shortens the relaxation region. A thin coating of dissolved sodium chloride in water on the shock tube wall, which on evacuation leaves a rough surface of crystals, however, does not seem to have any effect on the flow.