Electron transfer processes from semiconductor to molecular catalysts was studied in a model hybrid photo catalytic hydrogen evolution system com, posed of [Co-(III)(dmgH)(2)PyCl] (CoPy) and CdS under different pH conditions. Thermodynamic and kinetic studies revealed that photocatalytic H-2 evolution under high pH conditions (pH 13.5) can only account for the thermodynamically more favorable single-step simultaneous two-electron transfer from photoirradiated CdS to Co(III)Py to produce unavoidable intermediate Co(I)Py, rather than a two-step successive one-electron transfer process. This finding not only provides new insight into the charge transfer processes between semiconductors and molecular catalysts but also opens up a new avenue for the assembly and optimization of semiconductor molecular catalyst hybrid systems processed through multielectron transfer processes.