Cadmium metal, CO2, CO, and H-2 are produced under illumination of deaerated aqueous suspension of CdS in the presence of formate, The mechanism of cadmium metal photoformation and the influence of Cd-0 and the enzyme hydrogenase from Thiocapsa roseopersicina on the CdS-sensitized reactions have been studied. The reduction of the surface Cd2+ ions of the semiconductor lattice to Cd-0 is the result of the cooperative action of electrons of the conduction band and CO2.- anion radicals which are the primary products of formate oxidation by semiconductor holes. Up to 20% of Cd2+ of CdS can be reduced to Cd-0. Cadmium metal catalyzes the photoformation of molecular hydrogen. An alternative way of conversion of CO2.- results in the formation of CO and formate dehydration. Immobilized on the semiconductor particles, hydrogenase from T. roseopersicina activates processes of photodehydrogenation and photodehydration of formate sensitized by CdS. Zn the presence of cadmium metal and the enzyme, the rate of hydrogen photoproduction is up to 10 times higher than that catalyzed by Cd-0 alone. Cadmium metal photoformed on the surface of CdS participates in enzyme-catalyzed hydrogen photoproduction in two ways : as an activator of the transfer of the conduction band electrons to the enzyme active sites or as the substrate electron donor in the enzymatic hydrogen production.