The electrical properties of the silicon dioxide/n‐type (100) InP interface were significantly improved by thin interlayers of chemical bath deposited CdS. The CdS layer and CdS/InP interface were investigated with x‐ray photoelectron spectroscopy (XPS) and photoluminescence (PL). XPS data showed reduction of native oxides and the prevention of subsequent substrate oxide growth following CdS layer deposition. PL spectra, measured between 1.0 and 1.3 &mgr;m, indicate a reduction in phosphorus vacancies. Metal–insulator–semiconductor (MIS) capacitors fabricated with CdS‐treated InP substrates displayed interface‐state densities below 1×1011eV−1cm−2when determined from the difference between the high‐ and low‐frequency capacitance data.