We report the first experimental observation of a semiconductor/liquid junction whose open circuit voltageVocis controlled by bulk diffusion/recombination processes. Variation in temperature, minority‐carrier diffusion length, and/or in majority‐carrier concentration produces changes in theVocof then‐Si/CH3OH interface in accord with bulk recombination/diffusion theory. Under AM2 irradiation conditions, the extrapolated intercept at 0 K ofVocvsTplots yields activation energies for the dominant recombination process of 1.1–1.2 eV, in accord with the 1.12‐eV band gap of Si. A crucial factor in achieving optimum performance of then‐Si/CH3OH interface is assigned to photoelectrochemical oxide formation, which passivates surface recombination sites at then‐Si/CH3OH interface and minimizes deleterious effects of pinning of the Fermi level at the Si/CH3OH junction. Controlled Si oxide growth, combined with optimization of bulk crystal parameters in accord with diffusion theory, is found to yield improved photoelectrode output parameters, with 12.0±1.5% AM2 efficiencies and AM1Vocvalues of 632–640 mV for 0.2‐&OHgr; cm Si materials.