Photocatalysis is a rapidly expanding technology for wastewater treatment. In this review the chemical effects of various variables on the rate of degradation of different pollutants are discussed in detail. The effects of adsorption, temperature, intensity of light, pH, and the presence of anions, cations, etc have been specifically covered. A critical analysis of the available literature data has been made and some general conclusions have been drawn concerning the above mentioned effects. The need for more work on specific points has been brought out.