The reaction of theophylline with HO center dot radical, produced by photolytic methods at pH 7, was studied in aqueous solution and the products characterised by HPLC and GC-MS. In addition to the expected 1,3-dimethyluric acid, the formation of 1-methylxanthine and, to a lesser extent, of 3- methylxanthine was observed. Theoretical calculations confirmed the preferred formation of the former compound. Both demethylated products were also observed upon reaction of theophylline with O center dot- radical anion at pH similar to 13, and 1-methylxanthine was consumed faster than 3-methylxanthine after its formation. Molecular oxygen had no significant effect on the formation of the mono-methylxanthine derivatives. A reaction mechanism for the demethylation of theophylline by oxidising radicals is proposed. This demethylation reaction can play an important role in the protection of biological targets against oxidative stress as the first step of an antioxidising cascade.