A batch reactor model was used to predict the reactant concentration decay during constant current density (2000 A m(-2)) electrolysis of L-cystine hydrochloride to L-cysteine hydrochloride at a variety of cathode materials in 2 M HCl at 298 K. A divided parallel plate reactor of 1 dm(2) projected cathode area was operated in the batch recycle mode with a catholyte volume of 2 dm(3). The effect of the average linear flow velocity of the catholyte (0.033-0.123 m s(-1)) was studied and it was found that the system could be modelled as a batch reactor. At high reactant levels (i.e. at short times) the rate of reduction was under charge transfer control and at low reactant levels (i.e. at longer times) pure mass transport control ensued. The ability of the model to describe the experimental data was dependent on the hydrogen overpotential at the cathode material. A good fit was obtained at high hydrogen overpotential cathodes (such as mercury-plated copper and lead) but the fit was poorer at materials having a lower hydrogen overpotential (which include titanium, carbon, tin, stainless steel, copper, nickel and molybdenum).