On Pt/SiO2 catalysts, with the exception of EuroPt-1, rates of methyloxirane hydrogenolysis go through maxima at 33.3 kPa of hydrogen and the size of the maximum depends on platinum dispersion. Therefore, structure sensitivity is related not only to the clean metal surface, but also to the metal-hydrogen system. These relationships suggest that hydrogen- and/or methyloxirane-induced surface restructuring may occur and offer a method by which supported platinum catalysts may be characterized. Thus, carefully chosen chemical reactions distinguish among ambiguous and conflicting physical characterizations and are essential components in complete catalyst characterization. Additionally, optimum flexibility of 60%D particles explains the failure of EuroPt-1 to perform according to its dispersion and explains why diverse reactions exhibit rate maxima at approximately 60%D.