We report here the novel use of scanning electrochemical microscopy (SECM) to study photoelectron transfer (PET) kinetics at the semiconductor/electrolyte interface. The model system chosen consisted of a polycrystalline cadmium sulfide thin film in contact with methyl viologen (MV2+/+) and triethanolamine (TEOA) as redox mediator and hole scavenger, respectively. In the absence of illumination, SECM approach curves to CdS films deposited on glass showed negative feedback, consistent with an insulating substrate. Upon film illumination, positive feedback was obtained due to photoelectron reduction of MV2+ at the interface. The dependence of the pseudo-first-order rate constants (k(eff)) on light intensity and scavenger and mediator concentrations was estimated and the quantum efficiency as a function of light intensity was determined.