As a first step toward modeling the photoinduced repair of DNA, electronic structure calculations on the cleavage reaction of various pyrimidine dimers (uracil, thymine, and cytosine) as well as of their anion acid cation radicals have been carried out using the AM1 UHF method. Two different paths of the splitting reaction have been studied by locating all stationary points. Along the first path, the opening of the cyclobutane ring is initiated by breaking the C5-C5’ bond which leads to the formation of an intermediate, followed by the cleavage of the C6-C6’ bond; along the second path, the two C-C’ bonds are broken in reverse order. The results for the dimer anion radical favor a cleavage reaction along the first path while the second path is preferred for the cation radicals. Electron transfer to the dimers does not appreciably influence the enthalpy of the reaction for cycloreversion in the uracil and thymine dimers; however, it causes a dramatic reduction of the activation barrier for the cleavage reaction, In contrast, the reactivity of the cytosine dimer is only weakly affected by this electron uptake. Differences in the various reaction profiles are rationalized by invoking an energetic stabilization associated with the charge delocalization between fragments in the corresponding transition states. The calculated solvent effects evaluated by a dielectric continuum model show that the splitting reaction is sensitive to a polar environment. The reaction barriers of the splitting reaction are found to increase with the polarity of the medium, rationalizing the experimentally observed solvent effects on the dimer cleavage.