The electroreduction of Bi(III) ions in 1-8 mol dm(-3) chlorates(VII) in the presence of cysteine at dropping mercury electrode was studied using voltammetric and impedance methods. The determined kinetic and thermodynamic parameters indicated the magnitude of the catalytic effect of cysteine and allowed us to correlate these kinetic parameters with water activity. The presence of 1 x 10(-3) mol dm(-3) cysteine in the solution decreased the value of Delta S-0 at all the examined chlorate(VII) concentrations and suggested changes in the dynamics of the process of Bi(III) ion electroreduction. The multistage process is confirmed by the not rectilinear Ink(f)=f(E) dependences. In this mechanism, the chemical stages of Bi-Hg-2(SR)(2) or Bt-Hg(SR)(2) complexes formation with the participation of cysteine, which is a mediator in the formation of these complexes, and water can be distinguished. (C) 2012 Elsevier B.V. All rights reserved.