Thymine was placed in a model active oxygen-generating system containing ferrous sulfate, EDTA, and ascorbic acid. The oxidative products of thymine were separated by Sephadex LH-20 chromatography and a reversed phase high-performance liquid chromatography (HPLC) into at least five major components. One of them had a UV spectrum characteristic of 5-formyluracil and mass spectrometric analysis of this material also indicated this material to be 5-formyluracil.