The addition of acetate ion to an O-2-saturated mixed solution of acetonitrile and water containing oxalic acid as a reductant and 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA) as a photocatalyst dramatically enhanced the turnover number of hydrogen peroxide (H2O2) production. In this photocatalytic H2O2 production, a base is required to facilitate deprotonation of oxalic acid forming oxalate dianion, which acts as an actual electron donor, whereas a Bronsted acid is also necessary to protonate O-2(center dot-) for production of H2O2 by disproportionation. The addition of acetate ion to a reaction solution facilitates both the deprotonation of oxalic acid and the protonation of O-2(center dot-) owing to a pH buffer effect. The quantum yield of the photocatalytic H2O2 production under photoirradiation (lambda = 334 nm) of an O-2-saturated acetonitrile-water mixed solution containing acetate ion, oxalic acid and QuPh(+)-NA was determined to be as high as 0.34, which is more than double the quantum yield obtained by using oxalate salt as an electron donor without acetate ion (0.14). In addition, the turnover number of QuPh(+)-NA reached more than 340. The reaction mechanism and the effect of solvent composition on the photocatalytic H2O2 production were scrutinized by using nanosecond laser flash photolysis.