The hydride transfer from 1,5-dihydroriboflavin-2′,3′,4′,5′-tetra-acetate (FlH2) to various hydride acceptors (tetracyano-p-quinodimethane, tetracyanoethylene, and p-benzoquinone derivatives) in deaerated acetonitrile proceeds via electron transfer from FlH2 to hydride acceptors followed by proton and electron transfer. The formation of transient radical ion pair has been detected directly in the reactions of 1,5-dihydroriboflavin-2′,3′,4′,5′-tetra-acetate (FlH2) with some hydride acceptors in deaerated acetonitrile, providing unequivocal evidence for an electron transfer pathway in the overall two-electron redox reactions of FlH2 with hydride acceptors. The formal carbanion transfer from alkylcobalt(III) complexes to cobalt(III) porphyrin has also been shown to proceed via electron transfer, followed by the cleavage of cobaltcarbon bonds of alkylcobalt(IV) complexes and the subsequent bond formation between alkyl radical and cobalt(II) porphyrin to yield alkylcobalt(III) porphyrins. The rate constants of the overall carbanion transfer from alkylcobalt(III) complexes to cobalt(III) porphyrin agree well with those predicted by the Marcus theory for the rates of outer-sphere electron transfer reactions. In contrast with the case of alkylcobalt(III) complexes, the rate constants of the carbanion transfer from tetraalkyltin compounds (R4Sn) are 102-1013 times faster than those predicted by the Marcus theory depending on the size of alkyl group of R4Sn. The polar versus ET pathway is discussed in terms of the difference between outer-sphere versus inner-sphere electron transfer mechanisms based on the Marcus theory.