The effect of H-2 evolution on the mass-transfer coefficient of cathode reduction of potassium ferricyanide at a mercury cathode was studied with the aim of (i) comparing the mass transfer behavior at a Hg cathode with that at solid electrodes under gas-evolving conditions and (ii) testing the effect of anionic surface-active agent on the mass transfer behavior of an H-2-evolving Hg cathode. It is found that the mass transfer coefficient at the Hg cathode was much higher than the value at a solid cathode. The mass transfer coefficient at H-2-evolving Ha was found to decrease in the presence of sodium dodecyl sulfate (SIDS) surfactant by an amount ranging from 58.8 to 76%, depending on the H-2 discharge rate and the SDS concentrations. (C) 2003 Elsevier Inc. All rights reserved.