Single-potential-step chronocoulometry can be employed conveniently to study the kinetics and mechanisms of electrode reactions in the adsorbed state without complications by diffusion, provided the reactant is so strongly adsorbed as to permit the attainment of full coverage of the electrode surface at bulk concentrations as low as 10 −5 M. Under these conditions the mechanism can be disclosed in a straightforward and general way without making a priori assumptions as to the magnitude and coverage dependence of the surface activity coefficients of reactant and product, by carrying out measurements at constant surface coverage by the reactant. An application of this general procedure to progesterone electroreduction on mercury from aqueous solutions is provided. It is shown that in buffered solutions of 6 < pH < 7.5 progesterone (R) is electroreduced to the hydrodimer R 2 H 2 according to the mechanism: ▪ where AH denotes a proton donor.