In solutions of pure formamide, the mechanism of the electroreduction of Cd2+, Zn2+ and azobenzene, retarded by triphenylphosphine oxide (TPO) monolayers, is studied at the dropping Hg electrode. The size parameters of the activated complex and the inhibitor molecule show that the mechanism of retarded azobenzene reduction in formamide is similar to that of azobenzene in methanol and of p-aminoazobenzene in water. On the other hand, the mechanism of the inhibited reduction of Cd2+ and Zn2+ in formamide is similar to that in water but differs from methanol. These conclusions are correlated with the interaction parameters between the reactant and the adsorbate determined in this work.