Procedures have been developed for predicting direct photolysis rates of chemicals at lowconcentrations in air-saturated distilled water. Recently, attention has turned to the more complex problem of quantitating the effects of substances indigenous to aquatic environments on photolysis rates and products. These naturral substances influence photolysis rates of pollutants in aquatic environments through initiation of indirect photoprocesses, through the attenuation of sunlight, and through physical or chemical interactions that alter the speciation or microenvironment of pollutants. Recent research on these various effects is reviewed, with emphasis on studies of the influence of humic substances, suspended sediments, algae and natural inorganic species on photoreactions of trace organic chemicals.