The mechanism of sulfide and sulfoxide oxidation with peroxides (ROOH, R = H, Me), catalyzed by Mo (VI) complexes, was investigated by means of DFT/PBE1PBE calculations. Two different catalytic systems were considered: the first is based on the dioxocyclopentadienyl (Cp) complex CpMoO2Cl (Cp = eta(2)-C5H5), also active as a catalyst for olefin epoxidation, and the second based on MoO2Cl2. The most favorable mechanism in the Cp system is initiated by the O-H activation of the HOOR oxidant, which in the presence of CpMoO2Cl leads to formation of CpMoO(OH)(OOR)Cl. Although this is the active species for olefin epoxidation, an alternative pathway with lower energy is available. With the crucial H-bond assistance of another oxidant molecule, the oxoperoxo complex CpMoO(O-2)Cl forms, with release of alcohol ROH as byproduct and a calculated energy barrier below 25 kcal mol(-1). The mechanisms unveiled for sulfide to sulfoxide oxidation and for sulfoxide to sulfone oxidation are equivalent in their general features and follow outer-sphere mechanisms with S-nucleophilic attack from a free molecule of substrate (sulfide or sulfoxide) to the peroxide which is activated through Mo-O coordination. The MoO2Cl2 catalyst follows a similar course, calculated from MoO2Cl2(H2O)(H2O2). Again, explicit consideration of one molecule of solvent (water) proved essential to facilitate the H-transfer processes involved in the mechanism. The highest energy barrier calculated (ca. 25 kcal mol(-1)) corresponds to a H shift from the O-alpha to the O-beta atom of the coordinated H2O2 molecule, activating O-alpha for the oxidation reaction and preparing water (H2O beta) as the future leaving group. The outer-sphere mechanism ends with coordination of the oxidation product.