Differential capacity measurements at a mercury electrode in contact with aqueous solutions containing cetyltrimethylammonium cations, CTA(+) and supporting electrolytes of different types have been carried out. The results obtained are interpreted within the frames of the theoretical treatments presented in J. Phys. Chem. 96, 8453 (1992) and J. electroanal. Chem. 348, 59 (1993). It is shown that a surface film of aggregates starts to develop at positive polarizations. At pre-micellar concentrations this film is likely to be monolayer, whereas at concentrations around and above the cmc, the aggregation phenomena extend at least to two successive layers along the interface. The micellar film developed in this region covers the electrode surface throughout the negative polarization region. Depending on the potential this film undergoes a number of phase transformations. These transformations and more generally the properties of the film are affected by the presence of oxygen in the bulk solution and the nature of the supporting electrolyte. The desolved oxygen may affect the structure of bulk micelles and through them the structure of the adsorbed micellar film, whereas the possible role of counterions in the absorption process of CTA(+) is either direct, in the region where they are specifically adsorbed, or indirect, through the structure of the bulk micelles in the negative region where they are desorbed from the interface.