A comparative study of the complexes Mo2OxSy(cys)(2)(2-) (x = 4, 3, 2; y = 4 - x) as catalysts for electron transfer from irradiated TiO2 to acetylene is reported. These catalytic species all show similar behavior with respect to variations in intensity, loading, and pH. Their ability to facilitate electron transfer from TiO2 to substrate increases in the order Mo2O2S2(cys)(2)(2-) > Mo2O3S(cys)(2)(2-) > Mo2O4(cys)(2)(2-). At high light intensity, efficiencies for electron transfer from the colloidal TiO2 surface are comparable to those obtained with Pt under similar conditions. Efficiencies increase with decreased light intensity, giving product yields which account for 7-9% of the incident light at the lowest intensities studied. All three catalysts produce H-2 in N-2- or C2H4-saturated solutions, but only Mo2O2S2(Cys)(2)(2-) produces H-2 in the presence of C2H2. The effects of pH, temperature, intensity, and loading on the catalytic process are consistent with a proposed mechanism in which the catalytic site associated with the TiO2 surface promotes the transfer of electrons to the bound substrate. For each complex, a single catalytic site appears to be responsible for all reduction processes. A comparison of the photocatalytic properties of species containing the Mo(V)(2) core is included.