The results of kinetic measurements revealed an accelerating influence of cystine and water activity on multistep Bi(III) ion electroreduction in chlorates(VII). The magnitude of the catalytic effect of cystine was determined based on the obtained kinetic parameters, which were correlated with water activity. The multistep Bi(III) electroreduction process is controlled by the kinetics of the formation of active Bi-Hg(SR)(2) complexes preceding the transfer of consecutive electrons. Cystine is a mediator in the formation of these complexes, which are located inside the adsorbate layer. A mechanism is proposed for the catalytic action of cystine on Bi(III) ion electroreduction in chlorate(VII) solutions. (C) 2013 Elsevier Ltd. All rights reserved.