The adsorption of cytosine, 5-bromocytosine and native and denatured DNA was followed by measurement of the impedance of the electrode double layer. The two-dimensional condensation of both cytosine and 5-Br-cytosine and the formation of a self-assembled compact layer at the electrode surface was observed resulting in a capacitance pit on the C-E curves. The pit of 5-Br-cytosine occurs around the potential of electrocapillary maximum (ECM), the pit of cytosine around - 1.4 V. The electrode impedance is affected by the conformation of nucleic acid.