The formation of NADH radical cation in reactions with one-electron oxidants was observed for the first time. Transient products involving two tautomeric (keto and enol) forms of radical cation and neutral radical were spectroscopically characterized by means of pulse radiolysis. The kinetics of the decay of keto radical cation and neutral radical was investigated. The pK(a) value of the enol form of NADH radical cation was determined. The simple analogues of NADH and NAD(+), namely, 1-methyl-1,4-dihydronicotinamide and its oxidized form, were studied for comparison.