InP-based cells for photoelectrochemical light to electrical energy conversion are critically dependent on the oxidation state of the semiconductor interface. Cr(III)-Cr(II) electrolytes are favorably placed energetically for p-type cells but are lacking in charge transfer kinetics and prone to causing surface passivation by Cr-containing films. Moving from conventional (0.2M CaCl2) to saturated (5.6M CaCl2) electrolytes with the p-InP/Cr(III)-Cr(II) semiconductor-electrolyte interface provides a dramatic enhancement in electrochemical output and stability. Acceleration of electron transport for Cr(III) → Cr(II) conversion through ligand (Cl-) bridging and prevention of the formation of passivating Cr oxide films in the high chloride-lowered water activity medium are responsible for these beneficial effects. Comparison of the Cr couple's behavior to that of Eu(III)-Eu(II) and of Pt-catalyzed H2 evolution at metal and p-InP electrodes shows a much greater contrast for Cr(III)-Cr(II) voltammetry between the dilute and saturated CaCl2 electrolytes. Ring-disk electrode studies and cyclic voltammetry were used to establish the conclusions, and mass-transfer data (diffusion coefficients, kinematic viscosity) for these various Cr, Eu, and H species were obtained in the two media from rotating disk electrode measurements.