Density functional theory calculations using the hybrid functional B3LYP have been performed to probe the energetics of the spore photoproduct lyase (SPL) reactions. This enzyme catalyzes the repair of a thymine dimer caused by UV irradiation of bacterial spore DNA. The calculations support the experimentally suggested mechanism, in which the reaction proceeds through hydrogen atom abstraction from the C6 position of the thymine dimer, followed by beta-scission of the C-C bond linking the two bases. The calculations propose, furthermore, that an inter-thymine hydrogen atom transfer step takes place before the back-transfer of the hydrogen atom from the adenosine cofactor. The last step is shown to be the rate-determining step in the reactions.