Scalar-relativistic effects on the coarctate transition states (TS) for ethylene epoxidation with Mimoun-type diperoxo complexes [MO(eta (2)-O-2)(2)(OPH3)], M = Cr, Mo, W, were studied using gradient-corrected density functional theory at the BP86 level within the nonrelativistic, quasi-relativistic, and ZORA approaches. Spin-orbit effects were proved to be small. While scalar-relativistic effects scarcely influence the thermodynamics of the reaction with the tungsten complex, they shift the extent of reaction at the transition state toward the reactants and decrease the activation barrier significantly. The analysis of the transition states using a fragment-based energy-decomposition scheme shows that the reduction of the activation energy by relativistic effects is mainly due to the destabilization of the reactants rather than to the stabilization of the TS.