In this work were studied photochemical transformations of cysteine on model systems. It was synthesized chemical actinometer - potassium ferrioxalate, by which was determined the intensities of the lamps which were used as sources of radiation: lamp DRT-400 - 1, 98*10-8 einstein*s-1, lamp λ=254 nm - 7, 90*10-9 einstein*s-1 and for lamp λ=365 nm - 1,52*10-8 einstein*s-1. Cysteine was subjected to the photolysis: 1) directly, and it was found that the speed increases with increasing of the cysteine concentration. It was calculated the quantum yield and it was observed that is greater than unity, which means that only the first step is photochemically after cysteine generate radicals which lead to its oxidation; 2) induced, with OH radicals - generated by H2O2 and Cu2 + and Fe3 +, as the catalysts for the process of generation of OH radical, it has been found that the oxidation rate increases proportionally with increasing of cysteine concentration, ions of Cu2 + and Fe3 + in the system and more with increasing of the initial concentration of H2O2. 3) Sensitized, with the addition of humic acids. It was later proved that respect the same regularities as the direct and induced photolysis and humic acids can serve as sensitizers in the natural waters. Were calculated the effective constants of the oxidation, which are of the order of 10-4-10-5s-1 and were determined the mathematical formulas of the oxidation reactions. It was calculated the half-life and based on this parameter has been established that cysteine is subject to oxidation to the sources of radiation used, in the following order: τ1/2 (DRT-400)< τ1/2 (λ=365 nm)< τ1/2 (λ=254 nm)