A ubiquinone coenzyme analogue (Q(0): 2,3-dimethoxy-5-methyl-1,4-benzoquinone) was reduced by coenzyme NADH to yield the corresponding reduced form of Q(0) (Q(0)H(2)) in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex (1: [Ir-III(Cp*)(4-(1H-pyrazol-1-yl-kappa N-2)benzoic acid-kappa C-3)(H2O)](2)SO4) in water at ambient temperature as observed in the respiratory chain complex I (Complex I). In the catalytic cycle, the reduction of 1 by NADH produces the corresponding iridium hydride complex that in turn reduces Q(0) to produce Q(0)H(2). Q(0)H(2) reduced dioxygen to yield hydrogen peroxide (H2O2) under slightly basic conditions. Catalytic generation of H2O2 was made possible in the reaction of O-2 with NADH as the functional expression of NADH oxidase in white blood cells utilizing the redox cycle of Q(0) as well as 1 for the first time in a nonenzymatic homogeneous reaction system.