We develop a theory of charge transfer at semiconductor-catalyst interfaces to elucidate the current-potential behavior of semiconductor-catalyst-solution systems used for solar water splitting and compare simulations based on this theory to experimental data. Ion-permeable catalysts are found to form semiconductor-catalyst interfaces where the effective barrier height changes under operation yielding higher photovoltages and efficiencies relative to dense catalysts with the same catalytic activity. Such behavior is not captured by current equivalent circuit models, but is central to the study and design of efficient water-splitting systems.