Pyridine adsorption was used as a probe of the surface structure of supported molybdate and cobalt molybdate catalysts. The catalysts consisted of a γ-Al2O3support which had been impregnated with various molybdenum loadings ranging from 1.25% to 15% (MoO3). The Raman spectra of the catalyst and of the adsorbed pyridine indicated that a considerable portion of the alumina surface was not covered by molybdenum even after molybdenum trioxide was formed. Brnsted acid site and Lewis acid site adsorption were observed for low