Electrochemistry of various categories of intermediates was comparatively studied over wide range of electrode potentials, concentrations of proton donors (H3O+, NH4+), and temperatures. The suggested kinetic model considered two parallel pathways of electron transfer: either to an adsorbed intermediate (radical) or to the respective metastable complex with a proton donor. The electroreduction of all studied intermediates was found to obey the model. The formation of metastable complexes was shown to be facilitated for radicals having one or more functional groups. The particular reduction pathway is determined predominantly by the difference between overvoltages of electron transfer to a radical and to its metastable complex with a proton donor under the same experimental conditions (i.e. nature and concentration of a proton donor, electrode potential).