The effects of homocystine protonation on the kinetics and the mechanism of multi-step process of Bi(III) electroreduction in chlorates(VII) with varying Water activity has been studied. The methodology of the study is based on electrochemical methods: DC polarography, square wave voltammetric (SWV) and cyclic voltammetric (CV), as well as Electrochemical Impedance Spectroscopy (EIS). The highest catalytic activity of homocystine was stated in 2 mol center dot dm(-3) chlorates(VII) for the greatest amount of sodium chlorate(VII). The mechanism of the catalytic effects of HCY is associated with the formation of active Bi-Hg(SR)(2) complexes under specific conditions present on the electrode surface. (C) 2017 Elsevier B.V. All rights reserved.