A gas-evolving electrochemical cell with natural convection due to buoyancy is modeled using a hydrodynamic two-phase mixture model. Physical parameters are taken from the chlorate process, where hydrogen is evolved at the cathode. Constitutive closure laws, based on empirical relations developed for sedimenting particles, give the motion of the monodisperse gas phase relative to the mixture. Typical results display the effect of bubble size, channel width, and current density on the buoyant flow rate of the electrolyte through the channel. Results from the numerical simulations are also compared to data measured by Boissonneau and Byrne(1) on a cell with gas evolution on both the anode and the cathode. Qualitatively good agreement was found.