Gamma radiolysis of cysteine in aerated and oxygen saturated acidic solutions leads to a chain decomposition of the solute with a corresponding formation of cystine and cysteine sulphinic acid. A reaction mechanism to explain the experimental yields is proposed and a rate expression forG(-CySH) evaluated. It is found that the calculated and experimentalG(-CySH) are in excellent agreement. It is also found thatG(CyS-SCy) and G(H202) increase with decreasing dose rate (I).G(Cys-SCy) andG(H2O2) plotted against I/✓Iare straight lines suggesting a radical-radical chain termination step.