The effect of addition of sulphate to sodium chloride and sodium fluoride melts was investigated by chronopotentiometry in the temperature range 820-1000 degrees C. It was found that the numerical value of the chronopotentiometric term j tau(1/2)/c(0) increased with increasing cathodic current density. This behaviour was explained by chemical decomposition of sodium sulphate into SO3 and Na2O, followed by electrochemical reduction of SO3. The decomposition of sodium sulphate is enhanced with increasing temperature, resulting in an increase in the chronopotentiometric transition time. Numerical simulation of this electrode process supports this explanation of the electrode process. It is not a classical so-called CE mechanism when a chemical reaction precedes the electrochemical reduction as described in the literature, because the formation of SO3 is suppressed by the presence of Na2O in the melt. (c) 2006 Elsevier Ltd. All rights reserved.