Natural convection mass transfer at vertical and horizontal cylinders embedded in a fixed bed of Raschig rings was studied by an electrochemical technique which involves measuring the limiting current of copper deposition from acidified CuSO4 solution. Variables studied were Raschig ring diameter, physical properties of the solution and cylinder height in case of vertical cylinders or cylinder diameter in case of horizontal cylinders. Under the present conditions where high porosity beds were used, a slight decrease in the rate of mass transfer was observed in the case of vertical cylinders whereas no rate decrease was observed in the case of horizontal cylinders in the bed. Implication of the present results for the design of fixed bed reactors operating at low feed rates and containing a vertical or horizontal array of tubes for heat exchange with the bed was noted. (C) 2002 Elsevier Science B.V. All rights reserved.