We have investigated the kinetics of the photooxidation or photoreduction of 4-Nitrophenol with CdS as photocatalyst, and UV or visible light irradiation in the presence of Na2SO3 concentration under aerobic or anaerobic conditions, photocatalyst load and 4-Nitrophenol concentration. The 4-NP oxidation was attributed to the formation of the hydroxyl radical at 4 mM Na2SO3 concentration in either aerobic or anaerobic conditions under UV light irradiation. The 4-NPhenolate reduction was related to the sulfite radicals formed with appropriate Na2SO3 concentration (8mM) in anaerobic conditions under UV or visible light. The 4-NP oxidation and reduction reactions follow a zero order kinetic and can be used as a model reaction to evaluate the active photocatalyst under UV or visible light.