Sequential electron-proton addition to different NAD(+) model compounds was studied by pulse radiolysis. Under mildly acidic conditions, the radicals obtained by reduction of the cations are protonated at the carbonyl group. The pK(a) values of the resulting enol radical cations were determined. The same enol radical cations can also be formed via a deprotonation -reprotonation process from the keto radical cations of the corresponding NADH model compounds.