A two-phase mathematical model is applied to natural convection in an electrochemical cell. The model solves transport equations for both phases with an allowance of interphase mass and momentum exchange. The effect of current density and bubble size on the gas release rate, velocity field and void fraction distribution are investigated in a range of parameter. The flow in the system was generated due to the density difference between gas and liquid phases. It is found that both current density and bubble size significantly affect the gas release rate and velocity field. At an intermediate current density two circulation patterns form at the vicinity of the free surface. The circulations rotating opposite directions enhance lateral diffusion of gas phase. The gas evolution is enhanced with higher current density and lower bubble diameters. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.