A synthetic heme-Cu CcO model complex shows selective and highly efficient electrocatalytic 4e(-)/4H(+) O-2-reduction to H2O with a large catalytic rate (>105 s(-1))" While the hemeCu model (Fe Cu) shows almost exclusive 4e(-)/4H(+) r reduction of O-2 to H2O (detected using ring disk electrochemistry and rotating ring disk electrochemistry), when imidazole is bound to the heme (Fe(Irn)Cu), this same selective O-2-reduction to water occurs only under slow electron fluxes. Surface enhanced resonance Raman spectroscopy coupled to dynamic electrochemistry data suggests the formation of a bridging peroxide intermediate during O-2-reduction by both complexes under steady state reaction conditions, indicating that O-O bond heterolysis is likely to be the rate-determining step (RDS) at the mass transfer limited region. The O-O vibrational frequencies at 819 cm(-1) in 1602 (759 cm(-1) in 1802) for the Fe Cu complex and at 847 cm(-1) (786 cm-1) for the Fe(Im)Cu complex, indicate the formation of side-on and end-on bridging Fe-peroxo-Cu intermediates, respectively, during O-2-reduction in an aqueous environment. These data suggest that side-on bridging peroxide intermediates are involved in fast and selective O-2-reduction in these synthetic complexes. The greater amount of H2O2 production by the imidazole bound complex under fast electron transfer is due to 1e(-)/1H(+) O-2-reduction by the distal Cu where O-2 binding to the water bound low spin Fell complex is inhibited.