The competition between electrochemical dimerization and hydrogenation of the double bond in the reduction of activated olefins in water has been systematically investigated on the example of fumarodinitrile. The addition of a surfactant, here tetrabutylammonium paratoluenesulphonate, to the solution, suppresses the surface character of the protonation of the initially formed anion radical. The kinetic data were obtained using classical polarography in buffered medium as a function of the pH and of the substrate and buffer concentration. Analysis of the experimental data in terms of several theoretical models allows the following conclusions to be drawn as to the mechanism of the overall electrochemical reaction. The p K A of the protonated anion radical-anion radical couple appears to be >7.2. Protonation thus proceeds irreversibly. The dimerization reaction occurs essentially through the radical-radical coupling of two anion radicals. The interference of protonated anion radicals in the dimerization process, through self-coupling or coupling with anion radicals, if any, appears as a minor pathway. The protonated anion radicals tend to be reduced faster than they can dimerize. Their reduction occurs both by electrode-electron transfer and by homogeneous electron transfer, the reductant then being the anion radical.