The influence of the biologically important thiols cysteine, homocysteine and glutathione (reduced and oxidised forms) on the Cu(2+/0) system using a mercury monolayer carbon fiber electrodes was examined. Using fast cyclic voltammetry (v >= 1 kV s(-1)), the unusually rapid (approximate to 0.1 ms) electrolytic deposition of catalytically active copper atoms has been demonstrated. The catalytic analytical signal (AS) of the Cu(2+/0) system depends linearly on the thiol concentration over the range 0.1-10 mu M. The major factor influencing the AS was found to be the thiol's effective charge, which is pH dependent, enabling voltammetric discrimination of homocystine and the oxidised from of glutathione. The analytical applicability of the proposed catalytic procedure of thiol determination was confirmed by the convergence of the experimentally determined number of reduced and oxidized thiol groups of bovine serum albumin and white egg albumen compared with the calculated values. The procedure can also allow determination of free thiols. (c) 2009 Elsevier B.V. All rights reserved.