Methyl viologen (MV(2+)) gives a very narrow, sharp, tailless reversible peak by cyclic voltammetry at a mercury electrode. The peak arises from the one-electron reduction of the molecule to its cation radical (MV .(+)), which forms a two-dimensional (2D) phase over the electrode. In this work, we tested various analytical criteria in order to validate this assignation and fit the voltammetric peak numerically to the theoretical model developed for this purpose. The formation ofthe two-dimensional condensed phase of MV .(+) over mercury was found to depend on temperature. Like three-dimensional phases, 2D phases are not formed above a given, critical temperature. Thus, the voltammetric peak disappeared abruptly above 67 degrees C, which can therefore be adopted as the critical temperature for the 2D phase of methyl viologen.