A series of silica-aluminas with controlled composition has been synthesized and used as supports for Mo oxide catalysts. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were used to study the role of support composition upon the stability of various oxidic species, including products of solid state reaction between Mo oxide and silica-alumina. In general, the reducibility of Mo increases with increasing Mo loading, regardless of support composition. For a fixed Mo loading, however, extent of reduction increases with SiO2 content and shows a maximum at a composition SiO2 : Al2O3 = 75 : 25 (wt%). Except for silica-rich catalysts, the average oxidation state of Mo does not reach below +4 at the conditions used for TPR (maximum temperature : 550-degrees-C). Most of the support compositions yield XRD-amorphous Mo oxide. Only Mo-rich and silica-rich catalysts (MoO3 greater-than-or-equal-to 12 wt%; SiO2 greater-than-or-equal-to 50 wt%) contain crystalline phases of Mo : orthorhombic MoO3 on 100% SiO2, and MoO3 and Al2(MoO4)3 On silica-aluminas.