The electrochemical behavior at a mercury electrode of dopamine (DA) in the presence of citrate was investigated in aqueous solution, using cyclic voltammetry and alternating current polarography. The influence of the composition of the supporting electrolyte, citrate concentration and pH was evaluated. At potentials more positive than 250 mV DA and Hg are oxidized yielding, in the presence of citrate, dopaminoquinone (DQ) and Hg(II). Hg(II) reacts with the DA forming DQ, leucodopaminochrome (LDC) and dopaminochrome (DC). DQ undergoes further reduction at 200 mV. Citrate, in the form of Cit(3-), stabilizes the DC formed and this new electroactive compound suffers further reduction at -250 mV. This reaction pathway inhibits the formation of polymerized films on the surface of solid state electrodes and allows the determination of DA using the mercury electrode. (C) 2005 Elsevier Ltd. All rights reserved.