Size-quantized CdS thin films were prepared by sequential underpotential deposition of S and Cd on Au(111). The prepared films in an aqueous solution containing triethanolamine as a sacrificial electron donor showed n-type photoelectrode behaviors. Action spectra of anodic photocurrents were red-shifted with an increase of the film thickness toward characteristic values of those of bulk materials. The energy gap of the film determined from the action spectra showed a remarkable tendency of decrease with an increase of the film thickness up to 3.5 nm. Beyond 3.5 nm the rate of decrease with increase in the film thickness declined. An energy gap value characteristic of bulk materials was obtained with CdS films greater than 6.8 nm.