Modifications of the electronic structure of (112̄0) CdS surfaces as determined by Kelvin photovoltage spectroscopy have been related to changes in their atomic compositions as measured by Auger electron spectroscopy. Effects of chemical contamination, nonstoichiometry, and lattice disorder on the surface electronic properties could be separated by AES and low−energy electron diffraction for air−exposed, ion−bombarded, annealed, cleaved, and adsorbate−covered surfaces. Carbon contamination of CdS surfaces in ultrahigh vacuum is found to decrease photovoltage sensitivity. Argon ion bombardment and oxygen chmisorption produce nonstoichiometric surfaces. Ion−bombarded CdS exhibits a new pair of surface state levels which dominate the spectra of these surfaces. All of the photovoltage features attributed to surface state levels on (112̄0) CdS can be correlated with the chemical condition of the surface.