The transfer of the cationic forms of tryptamine and serotonin across the nitrobenzene (NB)/water (W) interface was studied by cyclic voltammetry. Well-defined voltammetric waves were observed within the potential window. The standard potentials of the transfer were determined from the midpoint potentials of the voltammograms. The transfer of the cationic form of tryptophan across the NB/W interface was also observed using an acidic aqueous solution. (C) 2011 Elsevier B.V. All rights reserved.