Cadmium sulfide (CdS) layers were deposited from an aqueous solution of thiourea, cadmium sulfate, and ammonia on (100)n‐InP at 60–95 °C. X‐ray photoelectron spectroscopy showed that the deposition process effectively removes native oxides on InP and forms a protective layer for subsequent dielectric deposition. Surface analysis also showed that the InP surface is not P deficient following oxide deposition on CdS‐treated InP. Capacitance–voltage and conductance–voltage measurements of metal–insulator–semiconductor (MIS) capacitors were used to compare samples with and without CdS films between InP and a deposited insulator. Capacitance–voltage response of CdS‐treated MIS structures showed well‐defined regions of accumulation, depletion, and inversion. The interface‐state density at midgap was reduced from 5×1011to 6×1010eV−1 cm−2with CdS treatment. Depletion‐mode MIS field‐effect transistors made using this new passivation technique exhibited superior device performance to that of untreated samples.