The electrochemical oxidation of aniline at low concentration was carried out in phosphate buffer solutions of pH 5. Both electrochemical and spectroscopic results indicate that during the initial stages of aniline oxidation, a dimerisation process takes place. The dimer has been identified as p-aminodiphenylamine (ADPA). No evidence supporting the formation of other dimerisation products, such as benzidine or hydrazobenzene has been found. The hydrolysis of ADPA seems to be a very slow process at pH 5 and, consequently, the formation of hydroquinone/p-benzoquinone has not been observed. ADPA is poorly soluble in aqueous medium and a part of these species precipitates on the electrode surface when formed. This precipitated dimer could act as an active centre for the further growth of the polymeric chains. Ministerio de Educación y Cultura (PB97-0130)