Far-UV irradiation of DNA leads to the formation of several types of dimeric lesions between adjacent pyrimidine bases including cyclobutane dimers, (6-4) photoproducts and Dewar valence isomers In the dry state, an additional specific thymine lesion, the spore photoproduct, is produced. We designed an HPLC-tandem mass spectrometry assay for the detection of the latter lesion. This technique that does not require radio-labelling of DNA allowed the simultaneous quantification of the spore photoproduct and other pyrimidine dimeric photoproducts. Using this approach, the complete distribution of bipyrimidine lesions within UVC-irradiated dry DNA was determined.