Rates of free convection mass transfer at horizontal screens made of woven cylindrical wires were,studied by measuring the limiting current of copper deposition from acidified copper sulphate solutions for a horizontal screen surrounded by a cylindrical counter-electrode. Variables studied were screens characteristic parameters and the physical properties of solutions. The data were correlated by a new mass transfer correlation. Mass transfer measurements at arrays of separated horizontal screens revealed that screen spacing has a. little effect on the rate of array mass transfer. On the other hand, increasing the number of screens per array was found to decrease the rate of array mass transfer. Implications of the present results for the design and operation of electrochemical reactors. suitable for heavy metal removal from waste water and electrosynthesis are noted.