The reaction pathways for thermal and photochemical formation of 5-methylcytosine (m(5)C) pyrimidine dimers (CPD) are explored using density functional theory techniques. It is shown that the methylation of cytosine does not contribute to an increased yield of CPDs after UV irradiation due to an even lower excitation energy at the reactant complex of m(5)C as compared to cytosine, a larger barrier to reach the decay channel corresponding to the transition state structure along the ground state reaction path, and a higher-lying decay channel. (C) 2004 Elsevier B.V. All rights reserved.