The electrochemical reduction mechanism of progesterone (P4) is studied in 0.1 mol dm(-3) N(C4H9)(4)PF6 + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave (SW) voltammetries as well as controlled potential bulk electrolysis at glassy carbon (GC) electrodes. The primary radical anion formed by the mono electronic reduction of P4 then undergoes a radical-substrate coupling to give a dimeric product which is also reduced and the dimeric dianion produced is then protonated by the solvent itself. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s(-1) and 10 Hz, respectively. Diffusion coefficient of P4 was calculated from convoluted cyclic voltammograms. Digital simulation was used to fit cyclic voltammetric responses. Formal potentials and formal rate constants as well as dimerization and protonation constants were evaluated from the fitting of cyclic voltammograms. The effect of scan rate and analytical concentration of the reagent on the electrochemical responses are also discussed. A general reaction mechanism is proposed. (C) 2008 Elsevier B.V. All rights reserved.