Concentration-dependent isotope effects (IEs) were measured in the competitive photocyanation of naphthalene and perdeuterionaphthalene in acetonitrile in the presence of electron accepters. The concentration of the reagents naphthalene, cyanide, and oxygen influences the magnitude of the IE. Experiments in which the photocyanation is brought about via selective excitation of an electron acceptor show that the naphthalene-concentration dependence of the Ifs is not caused by the naphthalene radical cations, which are intermediates in the photoreaction. This is corroborated by an electrochemical study of (the reactivity of) these radical cations. From these observations and complementary semiempirical PM3 calculations it is concluded that the variation of the IE with naphthalene concentration is due to an excited-state equilibration. A detailed reaction mechanism is proposed to account for the observed IEs.