A wide variety of optical properties can be found in the water bodies of the environment. These dramatic differences in composition, and thus in light absorption, have important effects on the dynamics and products of xenobiotic photoreactions in water. The paper reviews research showing that the absorption of sunlight by natural waters produces a variety of transient reactants that are involved in aquatic redox reactions. The review first examines two general approaches to studying photoredox kinetics--laser flash photolysis and continuous irradiations. Then, research results concerning some of the transient reactants are used to illustrate the application of these approaches. Among these transients are solvated electrons, triplet states and singlet oxygen, and superoxide ions and hydrogen peroxide.