The photovoltage variation with redox potentialERedis investigated onp‐InP andp‐CuInS2for various surface treatments. Etching and ion chemisorption are shown to successively improve CuInS2photovoltagesVPhresulting in a slope of ∼1 ofVPhvsERed. The respective densities of states are determined using a model calculation yieldingDs≊6×1011states cm−2 eV−1andDs≊1.2×1014states cm−2 eV−1for the unpinned and pinned situation, respectively. Good agreement between the calculated voltage drop in the Helmholtz layer due to CuInS2Fermi‐level pinning (FLP) and the missing photovoltage is obtained in a simple model calculation based on the surface state charge density. InP photocathodes show aVPhvsEReddependence in the dark, depending on pretreatment which is attributed to a chemical reaction that inhibits Fermi level equilibration. Positive from −0.4 V (SCE)VPhis independent ofERedfor polished and etched surfaces. The independence is explained assuming a dynamic equilibrium between semiconductor and solution without band‐edge shifting. At the potential wherep‐InP solar cells are operated (−0.47 V vs SCE), the electrodes behave as unpinned.