Radical ion pairs are generated in acetontrile solution by photoinduced electron transfer from naphthalene derivatives to cyanobenzenes and undergo reverse electron transfer to the singlet ground and to excited triplet states of the parent compounds. Both pathways lead to chemically induced nuclear polarization of the ground state products. They can be separated because the triplet contribution appears delayed by the triplet lifetime. Variation of the radical pair energy leads to a dominance of either the singlet or triplet channel. The analysis of the absolute net CIDNP effects and their time dependencies yields reaction probabilities for the reverse electron transfers which depend on the appropriate energy gaps as described by the Marcus theory, as well as rate constants for degenerate electron and spin exchange processes. For the donor/acceptor system naphthalene/(E)-1,2-dicyanoethene, the triplet pathway is also found dominant.