A new tandem base lesion has been identified in two DNA oligomers, namely d(GpT) and d(CpGpTpA), exposed to X-irradiation in deoxygenated aqueous solution. In this lesion the C6 carbon atom of thymine is hydroxylated and a covalent link is formed between the C5 carbon atom of thymine and the C8 carbon atom of the adjacent guanine base. In addition, further evidence in the form of mass spectrometric data is presented confirming the structures of previously reported tandem base lesions that are produced by ionizing radiation in the presence of oxygen. New data is presented on the prevalence of a previously reported tandem base lesion in which the methyl carbon atom of thymine is covalently linked to the C8 carbon atom of the adjacent guanine base. The free radical-initiated processes by which tandem base damages are generated are discussed. To date four different radiation-induced tandem base lesion have been identified. The evidence suggests that tandem base damage is a significant component of free radical-induced DNA damage.