The formation of condensed phases by salts of heptyl viologen cation radical on mercury electrode in aqueous media under potentiostatic conditions is explained in light of a new mathematical model based on a desorption-nucleation mechanism. This initially assumes the nucleation process to be governed by the desorption of molecules previously adsorbed on the electrode surface. The potential subsequent incorporation of adsorbed molecules into the condensed 2D phase via surface reorientation (a reorientation-nucleation mechanism) is considered. The proposed model was successfully applied to the 2D nucleation of heptyl viologen in the presence of both strongly (bromide) and weakly (sulfate) adsorbed anions on a mercury electrode surface.