A thiourea/ammonia pretreatment followed by chemical bath deposition of cadmium sulfide was used to passivate the surface of indium phosphide (100). The pretreatment was shown by X-ray photoelectron spectroscopy to effectively remove native oxides from the InP surface and form an indium sulfide layer. The subsequent chemical bath deposition of CdS on a sulfur-passivated surface forms a stable layer that protects the substrate from oxidation during chemical vapor deposition of SiO2. The passivation process was optimized for electrical response by varying the reactant concentrations of the chemical bath. Reflection high-energy electron-diffraction (RHEED) analysis showed that the pretreatment results in a (1 x 1) surface, which reconstructs to (2 x 1) after heating in vacuo to 200 degrees C. RHEED and atomic force microscopy showed an increase in CdS surface roughness with increasing thickness corresponding to the formation of oriented surface asperities. CdS-passivated metal-insulator-semiconductor diodes exhibited a density of interface states (D-it) of 10(11) eV(-1) cm(-2) when calculated by the high-low method, more than one order of magnitude lower than the D-it of untreated metal-insulator-semiconductor samples. A CdS layer thickness of similar to 10 Angstrom was determined to yield optimal capacitance-voltage response for all CdS deposition conditions investigated.