Photocurrent and differential capacitance were measured as a function of terminal potential with CdS‐ and GaAs‐electrolyte photovoltaic cells to determine the open‐circuit voltageVocand the power conversion efficiency &eegr;p.Vocwas found to be 1.2 V for both cells. With monochromatic excitation, &eegr;p=32% for the CdS cell, and &eegr;p=21% for the GaAs cell. For both cells the quantum efficiency was larger than unity at short wavelengths indicating that current doubling is occurring at the interface. For the GaAs cell, current flow is mostly due to the diffusion of minority carriers from the bulk to the space‐charge layer under illumination.