The kinetics and energetics of the electron transfer reactions occurring in the enzyme substrate complex of DNA photolyase have been studied by transient absorption spectroscopy with picosecond time resolution in the temperature range 275-90 K. The results can be summarized as follows: (i) The lifetime of (1)(FADH(-))* depends not only on the presence of the substrate but also on its nature, e.g. the electron transfer from (1)(FADH(-))* to thymine dimers T >$($) over bar T and to thymine-uracil dimers T >$($) over bar U is slower by a factor of 2 to 3 compared to that of U >$($) over bar U and U >$($) over bar T. This feature is attributed to the enlarged electronic coupling between (1)(FADH(-))* and the respective dimers in the absence of the methyl group in the C5 position of the 5'-uracil. (ii) The temperature dependence of the quantum yield was studied, and from this the activation energy for the overall repair process has been estimated to be E-A=0.45 +/-0.1 eV.