CdS/InP and CdS/GaAs heterojunctions were prepared by aninsituH2S/H2etching process and H2transport of CdS onto the InP and GaAs substrates. The etching involves a reaction of the III‐V compound surface to the corresponding sesquisulfides and simultaneous or subsequent removal by reduction in H2gas. The etching process changes the junction characteristics considerably. The junction onp‐GaAs shows a pinning of the Fermi level at the interface approximately 0.75 eV above the GaAs valence band. The junction onn‐GaAs is Ohmic.