The mechanism of the electroreduction Cd(II)/Cd(Hg) in n-methylformamide is studied under conditions of constant apparent electrode coverage by adsorbed triphenylphosphine oxide. The analysis makes use of distance parameters related to the wide potential Tafel plots and steric factors obtained from the relation between the rate constants and the inhibitor bulk concentrations corresponding to constant electrode coverage. Two possible mechanisms are discussed: (i) two one-electron reductions occurring at the vicinity of the outer Helmholtz plane and (ii) ion transfer followed by adsorption step with partial charge transfer. The possibility is also discussed that the position and the structure of the reactant at each step differs from low to high coverage.