Mononuclear copper complexes, [(tmpa)Cu-II(CH3CN)](ClO4)(2) (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)Cu-II(H2O)(2)](ClO4)(2) (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O-2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)(3)) in acetone, whereas 1 catalyzes the four-electron reduction of O-2 by the same reductant in the presence of Bronsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex [(tmpa)Cu-II(O-2)Cu-II(tmpa)](2+) , the two-electron reduced product of O-2 with Sc3+ is observed to be scandium peroxide ([ScIII(O-2 (2-))]+). In the presence of 3 equiv of hexamethylphosphoric triamide (HMPA), [ScIII(O-2(2-))]+ was oxidized by [Fe(bpy)(3)](3+) (bpy = 2,2-bipyridine) to the known superoxide species [(HMPA)3ScIII(O-2 (.-))](2+) as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O-2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex which is highly reactive toward O-2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O-2 following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O-2-reaction stoichiometry from four-electron with Bronsted acids to two-electron reduction in the presence of Sc3+ and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc3+ on copper-oxygen intermediates.