We present results of the growth of thin films of crystalline H2O2 and H2O2 center dot 2H(2)O (dihydrate) in ultrahigh vacuum by distilling all aqueous Solution of hydrogen peroxide. We traced the process using infrared reflectance spectroscopy, mass loss on a quartz crystal microbalance, and ill a few cases ultraviolet-visible reflectance. We find that the different crystalline phases-water, dihydrate, and hydrogen peroxide-have very different sublimation rates. making distillation efficient to isolate the less volatile component, crystalline H2O2.