Abstract MoO3, SiO2, and SiO2–MoO3 of different compositions were prepared and their surface and catalytic properties examined. A large number of acid sites were generated by mixing SiO2 with MoO3, while each component oxide by itself did not exhibit any appreciable acidic properties. The oxidizing property of MoO3 was enhanced by mixing with SiO2. No new compounds were detected in the oxide mixture by X-ray diffraction. The activity for depolymerization of 2,4,6-trimethyl-1,3,5-trioxane correlated well with the surface acidity. In the isomerization of 1-butene, both MoO3 and SiO2–MoO3 were active. The active sites on MoO3 were poisoned by CO2 but not by NH3, while those on SiO2–MoO3 were poisoned by NH3 but not by CO2.