Abstract: The photopolarization and wavelength response of single crystal CdS electrode in polysulfide and cyanide couple were measured. The conversion efficiency, fill factor and band gap were then calculated. Impedance spectra were also measured using a frequency response analyzer at various frequencies. The complex plane diagrams obtained were approximately equal to a semicircle, therefore,the interfacial structure can be described with asimple model.The Mott-Schottky plots show a good linearity. By extrapolating these straight lines to the potential axis the flat-band potentials were obtained which were quantitatively in agreement with potentials of photocurrent transition in photopolarization curves. A linear relationship was found between the flat-band potential of CdS electrode and the pH value of the redox couple. This relationship is explained in ferms of the formation of hydrated oxide film on the surface of CdS electrode in equilibrium with the H_aq⁺ ions in solution.