In aqueous media, heptyl viologen (HV2+) is reduced via a one-electron transfer to its cation radical (HV.+), the salt of which forms a two-dimensional (2D) phase on a mercury electrode. In this work, we studied the influence of temperature, the reagent concentration, and the type of anion present in the medium on the formation of this 2D phase. The critical temperature for the phase transition was found to be 80-85 degrees C. The experimental value obtained for the surface area occupied by the heptyl viologen molecule below 30 degrees C is about 70 Angstrom(2); accordingly, the molecules must arrange themselves at an angle with the electrode surface. The anion concentration in a chloride medium was found to exert no appreciable effect on the 2D phase; on the other hand, increasing bromide ion concentrations in the medium shifted the nucleation peak to increasingly more negative potentials.