A detailed study of the electrochemical reduction of benzil in aqueous medium between H-0 = -5 and pH 11 is presented. A global 2e(-), 3H(+) or 2e(-), 2H(+) surface reaction leads to cis and trans endiols. Analysis, using the theory of the square schemes with protonations at equilibrium, shows that in all the pH range the reaction is controlled by the second electron uptake. the paths being successively H+, e(-) and e(-), H+. The electrochemical reaction is followed by surface isomerizations whose rate constants vary from 7 to 200 s(-1) for the trans/cis is and from 1 to 200 s(-1) for the cis/trans transformations.