The kinetic characterizations of direct photolysis and indirect photoreactions in natural waters are described, including the reactions of multiple species in rapid reversible equilibrium. The photochemical sources, fates, and environmental effects of various photo-reactants are discussed. It is concluded that in the oceans, hydroxyl radical (*OH) and the aquated electron (eaq -) will ultimately oxidize and reduce, respectively, nearly all organic pollutants that are otherwise not degraded before transport to the oceans. In atmospheric, and surface waters (fresh and saline), Superoxide radical ion (•O2 -) can be involved in the oxidation of reduced forms of transition metals (e. g. Cu (I), Fe (II)), and for certain transition metals (e. g. Cu(I)) can also be the dominant source of the reduced form of the transition metal. In atmospheric waters and probably also in surface (fresh and saline) waters, hydrogen peroxide (HOOH) is a significant source of hydroxyl radical, through the iron photo-Fenton’s reaction. Hydrogen peroxide is the single most important oxidant for oxidizing sulfur dioxide to sulfuric acid during periods of cloudiness. Excited state triplets and organic peroxyl radicals derived from natural organic chromophores play significant roles for the oxidation of phenols in natural waters.