H-1-NMR and X-band EPR were used to study the second-order rate constant for the electron transfer reactions involving naphthalene (C10H8) and perdeuterated naphthalene (C10D8) in the absence of ion association. In hexamethylphosphoramide the second-order rate constant for the intermolecular electron transfer from C10D8.- to C10D8 [(6.4 +/- 0.2) x 10(8) M(-1) s(-1)] is about 20% larger than it is for C10H8.- + C10H8 = C10H8 + C10H8.- (k(H-H)/ k(D-D) = 0.80). This is explained in terms of Marcus theory and the known 0.012 Angstrom bond shortening that takes place when H is replaced by D in C10H8 and an estimated 0.02 Angstrom bond shortening when H is replaced by D in C10H8.-. The rate constants for electron transfer from C10D81- to C10H8 [(9.8 + 0.1) x 10(8) M(-1) s(-1)] and from C10H8.- to C10D8 [(3.5 +/- 0.6) x 10(8) M(-1) s(-1)] show that the electron transfer rates increase as the donor naphthalene is deuterated and the acceptor is protonated.