The diffusion-mediated quenching rate constant kq associated with photoinduced electron transfer (ET) reaction between cations in aqueous solutions containing inert electrolyte was theoretically studied by using the modified Tachiya-Murata theory [J. Phys. Chem. 96 (1992) 8441]. The ET was assumed to occur not only at the contact distance R between the reactants but over a range of distances r > R. The calculated kq and its dependence on the ionic strength are in reasonable agreement with experimental values obtained from the single-photon counting measurements on the system 9-anthrylmethylammonium cation donor (D+)-methylviologen cation acceptor (A2+).