We have studied the detailed reaction mechanism of iron and manganese superoxide dismutase with density functional calculations on realistic active-site models, with large basis sets and including solvation, zero-point, and thermal effects. The results indicate that the conversion of O2- to O2 follows an associative mechanism, with O2- directly binding to the metal, followed by the protonation of the metal-bound hydroxide ion, and the dissociation of 3O2. All these reaction steps are exergonic. Likewise, we suggest that the conversion of O2- to H2O2 follows an at least a partly second-sphere pathway. There are small differences in the preferred oxidation and spin states, as well as in the geometries, of Fe and Mn, but these differences have little influence on the energetics, and therefore on the reaction mechanism of the two types of superoxide dismutases. For example, the two metals have very similar reduction potentials in the active-site models, although they differ by 0.7 V in water solution. The reaction mechanisms and spin states seem to have been designed to avoid spin conversions or to facilitate them by employing nearly degenerate spin states.