An EPR study of Mo5+ species formed on photoreduction of MoO3---SiO2 catalysts in H2 is reported. The major product of photoreduction at -253 °C is a tetrahedrally coordinated Mo5+OH- species showing 1H hyperfine splitting formed by reaction of H2 with a hole—electron pair. At higher temperatures this is converted to an octahedral Mo5+OH- also showing 1H hyperfine splitting, and ultimately to a distorted octahedral Mo5+O2- species which has no bound protons and which resembles the Mo5+ formed on thermal reduction of the same catalysts in H2 above 300 °C. A CO adduct of the tetrahedral Mo5+OH- has been characterized by its EPR spectrum. The chemistry of photoreduction is compared with that of thermal reduction, and it is suggested that both may occur via similar pathways.