Bimetallic Pt-Mo catalysts deposited on silica show unusual behavior when used in reactions of hydrocarbons with hydrogen (hydrogenolysis, isomerization, dehydrocyclization). This can be explained by variations of the surface composition, i.e., the Mo/Pt surface ratio, with the conditions, particularly the temperature and the composition of the gas phase in contact with the catalyst. Two methods for detecting such variations have been used in this work : these are XPS analysis and the product distribution in 3-methylpentane hydrogenolysis which is very sensitive to the surface composition. Both methods have shown that the surface composition of Pt-Mo/SiO2 catalysts is modified in the presence of aromatic hydrocarbons, the molybdenum surface content of metal particles being decreased compared with their original molybdenum content after reduction in hydrogen.