An experimental study has been made of photovoltaic effects which occur at semiconductor-electrolyte interfaces. Single crystal specimens of CdS and several other compounds were used. It was found that in a number of cases the photovoltaic effect results from a chemical reaction of the electrode materials. In such cases the observations may be explained by a simple mechanism which relates the sign of the photo-emf to the conductivity type of the semiconductor and to the chemical reaction which is occurring. The reaction may be predicted using readily available thermodynamic data. A different process occurs when the electrode material is GaAs. It acts as an inert electrode which exchanges electrons with an oxidation-reduction couple in the solution.