The electrochemical oxidation of aqueous wastes containing 4-nitrophenol (4-NP) using a boron-doped diamond thin-film electrode has been studied. Within the parameter ranges used (1504000 mg of 4-NP dm(-3), pH 2-12, 30-60 mA cm(-2), 25-60degreesC), the complete treatment of the organic waste was achieved. The maximum current efficiencies were obtained under kinetic control. On the basis of the results of voltammetric and galvanostatic electrolysis studies, a simple mechanistic model was proposed. The first stage in the treatment of 4-NP-containing aqueous wastes is the release of the nitro group from the aromatic ring. As a consequence, phenol or quinones are formed. These organic compounds are oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. On the cathode, the reduction of the 4-NP to 4-aminophenol takes place. In alkaline media, this compound can be polymerized and transformed into a dark brown solid.