The influence of the cation of the base electrolyte and its concentration (0-1 mol dm-3) on the oxidation of ferrocyanide to ferricyanide in aqueous solutions is investigated using platinum microdisc electrodes. It is shown that increasing the concentration of all the cations causes the formal potential for the couple to shift to more positive potentials (i.e. the cations stabilize the more highly charged reduced species preferentially). Also, the formal potential in solutions of a strontium salt is always positive to that in one containing the same concentration of potassium ions, implying stronger ion association by the divalent ion. In addition, it appears that all cations catalyse the heterogeneous electron transfer reaction, this effect increasing along the series Li + < Na + < K+ < Sr2+. It is confirmed, however, that under some conditions, ferricyanide decomposes on the surface of the platinum leading to blocking of the surface and, indeed, there appears to be a correlation between the conditions for surface blocking and those where the kinetics of the electron transfer reaction are diminished. Hence, it is proposed that the cation effect on the kinetics of electron transfer results from a change in the extent of surface blocking. This could arise if ion association of the ferro/ferricyanide with cations prevents or decelerates their decomposition to the species blocking the surface.