60Co-gamma radiolysis of 3-iodotyrosine and 3,5-diiodotyrosine in aqueous-ethanol solutions has shown that the chemical effects are mainly determined by the interaction of radicals from the radiolysis of solvent and controlled by the composition of the solution. The influence of varying solvent composition and radiation dose on the amount of iodoamino acid converted and on the yields of the radiolysis products (I2, I–, IO
and H2O2) formed in aerated solutions at room termperature were investigated. The formation of I2 is dependent upon the acidity of the solution and is mainly produced as an after-effect due to the interaction of H2O2 with I– ions, both being radiolysis products. The variation of radiation-chemical yields with the solute and solvent composition, and the probable mechanisms for formation of the radiolysis products are discussed.