Natural convection mass transfer at a vertical array of closely-spaced horizontal cylinders was studied by an electrochemical technique involving the measurement of the limiting current of the cathodic deposition of copper from acidified copper sulfate solution. Various combinations of solution concentration, cylinder diameter, and number of cylinders per array were used including experiments on single cylinders. Results for single cylinders are correlated in the range 1.5 x 10(7) < Sc . Gr < 1.3 x 10(10) by the equation Sh = 0.231(Sc . Gr)(0.283). The transfer coefficient at the array was found to decrease with increasing number of cylinders, pass through a minimum, and then increase with further increase in the number of cylinders per array; the mass transfer coefficient increased with increasing cylinder diameter in the array. Mass transfer data for different arrays were correlated for the range 6.3 x 10(9) < Sc . Gr < 3.63 x 10(10) by the equation Sh = 0.455(Sc . Gr)(0.25) and for the range 6.3 x 10(10) < Sc . Gr < 3.63 x 10(12) by the equation Sh = 0.0064(Sc . Gr)(0.42). The characteristic length used in the above correlations was obtained by dividing the array area by the perimeter projected onto a horizontal plane. Practical implications of the present results in designing electrochemical reactors with heat transfer facilities are highlighted.