The monolayer films of phenothiazines, methylene blue (MB) and thionine (TH) were prepared and characterized by cyclic voltammetry. The maximum surface concentrations in monolayers at interface mercury electrode/organic film were determined. The properties of monolayers are influenced by the type of anion in the solution. The phase transformation in the thionine monolayer is caused by the decrease of temperature. In the case of methylene blue it is induced by the presence of nitrate due to its specific interaction with MB molecules.