The magnitude of the hydrogen-deuterium isotope effect (IE) in the photosensitized cyanation of biphenyl is found to depend on the concentration of biphenyl and on that of cyanide, as well as on the ionic strength of the medium. These concentration dependences are shown to originate from intermolecular electron exchange between biphenyl and its radical cation, in competition with addition of cyanide. Similar observations are made for the three symmetrically substituted dimethylbiphenyls upon deuteration of the methyl groups. The overall magnitude of the IE is determined by the IE of the back electron transfer step within the initially formed ion pair, as well as by the IE of the electron exchange equilibration. The experimentally determined equilibrium IEs for the biphenyls are in agreement with the results of semiempirical and ab initio calculations.