The steady-state gamma radiolysis of deaerated aqueous solutions of thymidine generated a complex mixture of pyrimidine and nucleoside derivatives. Twenty-two of these compounds have been isolated and unambiguously characterized by spectroscopic methods including proton nuclear magnetic resonance and mass spectrometry. The major 5,6-saturated products has been identified as the 5R and 5S diastereoisomers of 5,6-dihydrothymidine and their mono and dihydroxylated derivatives on the 5 and/or 6-carbons. The G values of these various compounds has been determined. The roles of the primary reactive species derived from the radiolysis of water have been studied by using specific radical scavengers i.e., ethanol, t-butanol and potassium nitrate.