In sunlit surface waters, aquatic humics and nitrate act as sensitizers or precursors for the production of photoreactants such as singlet oxygen, humic-derived peroxy radicals, hydrogen peroxide, and solvated electrons. Lifetimes of the various reactants are controlled by their reactions with aquatic humics, by solvent quenching (singlet oxygen), by reactions with molecular oxygen (solvated electron), or by other processes (peroxy radicals). The steady-state concentration of each transient formed during solar irradiation was determined from the observed disappearance rate of added organic probe compounds. The probe compounds have selective reactivities with the individual transient species of interest. Effects of these photoreactants on the elimination of micropollutants and on chemical transformations of dissolved organic matter are discussed.