Olefin epoxidations are a class of reactions appropriate for the investigation of oxygenation processes in general. Here, we report the catalytic epoxidation of various olefins with a novel, cross-bridged cyclam manganese complex, Mn(Me2EBC)Cl-2 (Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), using hydrogen peroxide as the terminal oxidant, in acetone/water (ratio 4:1) as the solvent medium. Catalytic epoxidation studies with this system have disclosed reactions that proceed by a nonradical pathway other than the expected oxygen-rebound mechanism that is characteristic of high-valent, late-transition-metal catalysts. Direct treatment of olefins with freshly synthesized [Mn-IV(Me2EBC)(OH)(2)](PF6)(2) (pK(a) = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo-Mn-IV nor oxo-Mn-V species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the 180 labels in an acetone/water ((H2O)-O-18) solvent demonstrate that no O-18 from water ((H2O)-O-18) is incorporated into epoxide products even though oxygen exchange was observed between the Mn-IV species and (H2O)-O-18, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, O-18(2), and hydrogen peroxide, (H2O2)-O-18, confirm that an oxygen atom is transferred directly from the (H2O2)-O-18 Oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn-IV(Me2EBC)(O)(OOH)(+), was detected by mass spectra in aqueous solutions prepared from Mn-II(Me2EBC)Cl-2 and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn-IV(Me2EBC)(O)(OOH)(+), is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.