The non-volatile products formed by γ-irradiation of DL-mandelic acid in 0·1% oxygenated, aqueous solution have been determined quantitatively by isotope dilution analysis. The initial products of irradiation are carbon dioxide, phenylglyoxylic acid, and benzaldehyde, part of which is converted by autoxidation into benzoic acid. Phenols resulting from hydroxylation of the aromatic nucleus are also observed but the principal abstractions occur in the side-chain. Mechanisms to account for these products are suggested.