The influence of homocysteine on double layer parameters at the electrode/chlorates (VII) interface and multi-step Bi (III) ion electroreduction was investigated using both voltammetry and impedance methods. The catalytic activity of homocysteine increases with an increasing amount of NaClO4 in a basic electrolyte solution. The highest catalytic activity of homocysteine was observed for 2 mol.dm(3) chlorates (VII). The multistage process is confirmed by the not rectilinear Ink(f) = f (E) dependences. In this mechanism, the chemical stages of Bi-Hg(SR)(2) complexes formation with the participation of homocysteine, which is a mediator in the formation of these complexes, and water can be distinguished. The composition of these complexes is diversified in solutions that differ in the degree of amino acid protonation. (C) 2016 Published by Elsevier Ltd.