Gradual passivation of the working electrode by species generated in the electrochemical reaction of the analyte often results in severe signal instability in the voltammetric detection of organic compounds. Using six test analytes which exhibit this behaviour (phenol, tyramine, tyrosine, 5-hydroxytryptamine (scrotonin), tryptophan and 3,4-dihydroxyphenyl acetic acid (DOPAC)) it was found that the fall-off of the oxidation signal of the analytes during repeated square-wave voltammetric scans was greatly reduced or even eliminated by the cationic surfactant cetyl trimethyl ammonium chloride (CTAC) and the anionic surfactant sodium dodecyl sulfate (SDS). The surfactant concentration was 5000 mg L-1 which is above the critical micelle concentration of CTAC as well as of SDS. The working electrode was glassy carbon. The signal stability of the analytes improved with the strength of the phosphate buffer used as electrolyte medium in the absence as well as in the presence of the surfactants. The experimental results are interpreted in terms of the competitive adsorption equilibria at the electrode surface and the aggregation behaviour of the surfactants. (c) 2006 Elsevier B.V. All rights reserved.