Acidity of the oxidic molybdenum catalysts supported on mixed ZrO2-SiO2 and ZrO2-Al2O3 carriers was investigated by Fourier transform infrared spectroscopy of pyridine adsorption. Deposition of molybdenum on ZrO2-SiO2 and ZrO2-Al2O3 supports leads to formation of surface Brønsted acid sites. The number of the Brønsted and Lewis acid sites in supported-molybdenum catalysts depends on both the ZrO2 content and the type of the support. With increasing ZrO2 content, the Lewis acid sites increase for both series of catalysts. The Brønsted acid sites are higher for Mo/ZrO2-SiO2 samples compared to those for Mo/ZrO2-Al2O3 and also increase with zirconia.