The interfacial micellization of a homologous series of N-alkyl pyrimidinium salts and their transformations on a Hg electrode are studied by means of differential capacitance measurements. It is shown that below the cmc a monolayer film of surface micelles is formed at positive polarizations, around + 0.15 V with respect to the saturated calomel electrode, and covers the electrode surface up to -1.2 V where this film is destroyed due to the onset of a reduction of N-alkyl pyrimidinium cations. The product of this reduction is strongly adsorbed on the electrode surface and undergoes a surface phase transition forming a compact layer in the region between -1.3 and -1.7 V. At concentrations above the cmc the picture is almost the same except the fact that in the polarization region from ca. +0.2 to +0.1 V the micellar film collapses to a compact layer.