The model of mass transport taking into account mobile chemical interactions in the Nernst-type diffusion layer is used for the estimation of surface concentration of Sn(II) and Co(II) citrate complexes. The analysis of the solution composition indicates that the complexation degree of Sn(II) is much higher than that of Co(II) in the ligand-deficient slightly acid media. This fact makes it possible to choose optimal concentrations of solutions wherein equilibrium potentials of Sn and Co are close. The kinetics of SnLH- reduction in cobalt-free solutions may be described quantitatively within the framework of the model with cathodic alpha = 0.7 and the exchange current density i(o) = 6 mA cm(-2). A similar mechanism provides the best fit for a partial process of Co(II) reduction in solutions containing Sn(II). The effect of Co(II) on the kinetics of Sn(II) reduction may be interpreted on the assumption that SnL2- species may also be treated as an electrically active complex. (C) 2000 Elsevier Science Ltd. All rights reserved.