In aqueous media, the cation radical of heptyl viologen, HV.+, forms a two-dimensional (2D) phase on a polycrystalline silver electrode. The phase results from the reduction of HV2+ molecules adsorbed on the electrode and consists of the cation radical salt and bromide as counterion; it is dissolved through gradual nucleation and growth of vacancies, which take place simultaneously with a Langmuir-type desorption process.