The adsorption of cesium ions from aqueous sulphide solutions on semiconductors belonging to the II–VI family of compounds was investigated using Rutherford backscattering in the random and channeling direction, and proton‐induced x‐ray emission, with computer simulation of the spectra. Whereas CdSe and CdS are stable against surface exhange (in the dark) ZnSe is unstable in such solutions and extensive O/Se and S/Se exchange occurs after relatively short periods of immersion. Furthermore, while cesium ions are chemisorbed mostly on the semiconductor surface of CdSe and CdS with some penetration into the crystal matrix, extensive indiffusion of cesium occurs in ZnSe. These results may explain the difference in the behavior of these two kinds of materials in sulphide electrolyte under illumination. They can also explain the fact that in ZnSe the work function of the semiconductor does not vary appreciably upon immersion in cesium containing solution while for CdSe and CdS the work function decreases by 150–200 mV and consequently the open circuit potential increases by that amount in cesium sulphide solutions.