In aqueous mixed media containing DMSO in fractions by volume above 0.50 the cation radical of heptyl viologen (HV.+) can be absorbed in two different orientations on a mercury electrode. Thus, the cation radical can take a planar conformation relative to the surface; however, depending on the applied potential, it can reorient itself to adopt a different conformation where the bipyridine group lies normal to or at an angle with the electrode. In this work, the reorientation was found to take place via a two-dimensional phase transition mechanism. The experimental conditions under which the phenomenon occurs are analyzed.