The catalytic influence of methionine on the mechanism and kinetics of Bi(III) ions electroreduction in chlorates (VII) solutions with varied water activity at dropping mercury electrode in chlorate (VII) using voltammetric and impedance methods was studied. The determined kinetic parameters pointed at the magnitude of catalytic effect of methionine and allowed to correlate these kinetic parameters with water activity. It results from the values of transfer coefficients, standard rate constants and activation energy that methionine accelerates the electroreduction process of Bi(III) ions in 1-8 mol dm(-3) chlorates (VII). The highest catalytic activity of methionine was observed in 4 mol dm(-3) solution of chlorates (VII). The character of rate constants changes in a function of potential points at the existence of multi-stage electroreduction process of Bi(Ill) ions in the presence of methionine as well as the different mechanism of Bi(III) electroreduction in the presence of methionine in the solutions with low water activity comparing to the solutions with high water activity. (C) 2011 Elsevier B.V. All rights reserved.