The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)33+ (bpy = 2,2'-bipyridine). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d2) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH*+ were determined from the kinetic analysis of the electron-transfer reactions.