The relationship between the mass transfer coefficient and the shear stress along the vertical electrode was investigated under electrolytic gas evolution, either oxygen or hydrogen, from alkaline solution containing ferrecyanide and ferricyanide ions. The shear rate obtained from the shear stress measurement was empirically correlated with the gas evolving current density, the electrode height and the liquid kinematic viscosity. The dimensionless correlation of the mass transfer coefficient with the shear stress under oxygen evolution condition agreed formally with the correlation of the mass transfer in turbulent-free convection. On the other hand, the experimental results under hydrogen evolution varied greatly from those under oxygen evolution. NOMENCLATURE surface area of electrode, cm* concentration in bulk, mol dm-" diffusivity, cm* s-' Faraday constant, C mol -' dimensionless velocity gradient Grashof number (= gp(AT)L"/v') gravitational acceleration, cm s-s heat transfer coefficient, W crn2K-' dimensionless concentration gradient limiting current of reactant ion, A gas evolving current, A total current of mass transfer and gas evolution (=4+1&A limiting current density of reactant ion, A cm-r gas evolving current density, A cn-* mass transfer coefficient, cm s-l electrode length, cm Nusselt number number of charge transfer Prandtl number Schmidt number Sherwood number temperature difference, K dimensionless velocity head due to buoyancy electrode weight at gas evolution, g electrode weight at zero current, g electrode weight at mass transfer and gas evolution, g distance along electrode surface, cm den&cation coefficient, K -1 thermal conductivity, W cm-' K-' viscosity of liquid, g cm-' s kinematic viscosity of liquid, cm* s-' density of liquid, g cm-' shear stress, g cm-' s-'