Molybdena-silica catalysts have been characterized spectroscopically and their thermal reduction in H2 studied by spectroscopic and volumetric measurements. Infrared spectroscopy complements other techniques in identifying MoO3 and surface polymolybdate phases in the calcined catalysts. The polymolybdate phase is readily hydrolyzed on exposure to water vapour. Initial hydrogen reduction occurs at 400 °C in the polymolybdate phase, producing Mo5+ and uncovering the silica support. At 500 °C, terminal oxide vacancies are formed in both polymolybdate and MoO3 phases.