Factors affecting the rate of liquid-solid diffusion controlled reactions at gas evolving fixed bed reactor were studied by measuring the mass transfer coefficient of cathodic copper deposition from acidified CuSO4 above the limiting current where simultaneous H-2 evolution takes place in a reactor packed with either cylinders or Rasching rings with no net electrolyte flow. Variables studied were particle size, bed height and H-2 discharge velocity. The rate of mass transfer was found to be proportional to the square root of H-2 discharge rate. Bed height and particle size were found to have a negligible effect on the rate of mass transfer. For a given set of conditions the mass transfer coefficient at a fixed bed of cylinders is higher than that at a fixed bed of Raschig rings. A mathematical model based on bombardment of the mass transfer surface by high kinetic energy bubbles is presented to explain the results. Mass transfer data at the cylinder bed reactor were correlated by the equation J = 1.58(Re.Fr)(-0.162) While the data at a fixed bed of Raschig rings were correlated by the equation J = 1.4(Re.Fr)(-0.162) Practical applications of the results in the design and operation of fixed bed electrochemical and catalytic reactors used in wastewater treatment were highlighted. (C) 2011 Elsevier B.V. All rights reserved.