Photoinduced electron-transfer (ET) reactions of Ru(CN)(2)L(2) (L = bpy and phen derivatives) with a series of Ru-III (beta-diketonato)(3) were investigated in acetonitrile solutions and compared with organic quencher systems (TCNQ, benzoquinone, and nitrobenzene, etc.). In the normal and the plateau region (Delta G > -1.2 eV), the quenching rate constants of Ru(beta-diketonato)(3) are smaller than that of organic molecules at the same Delta G. The disadvantage of the ET of the Ru(beta-diketonato)(3) quenching system arises mainly from the low transmission coefficient caused from the weak orbital overlap between donor and acceptor. Although a slight decrease of quenching rate was found for each donor in the highly exothermic region (Delta G < -1.6 eV) in which Ru(hfac)(3) was used as a quencher, evidence of the quadratic inverted region was not obtained. Some reasons for the weak Delta G dependence of k(q) in this region are discussed.