The fluorescence quenching electron-transfer (ET) and the back ET within geminate radical ion pairs (GRIP) were studied in benzonitrile (BN) using anthracenecarbonitriles as electron-accepting fluorescers and amino-, methyl- and methoxybenzenes as electron-donating quenchers. A direct evidence that the long-distance ET fluorescence quenching takes place at the diffusion-controlled limit was found for the first time. Furthermore, the present results were compared with the previous ones obtained for the same fluorescer and quencher pairs in acetonitrile (AN) in order to examine whether the photoinduced ET reactions can be affected by the kind of solvent, i.e., aliphatic and aromatic, and particularly to make clear whether there happens to be a pi-electron-mediate superexchange ET in BN. The free energy dependence of fluorescence quenching rate constant (k(q)), effective quenching distance (r(q)), free radical yield (Phi(R)), and rate constant (k(bet)) of back ET within GRIP in BN was the same as that in AN as long as the differences in dielectric constant, diffraction index, and viscosity between BN and AN were taken into account. The fact that there is no particular difference between photoinduced ET reactions in AN and BN evidences the absence of pi-electron-mediate superexchange ET in BN.