The free convection mass transfer behavior of a fixed bed of spheres has been studied experimentally using an electrochemical technique which involved measuring the limiting current of the cathodic deposition of copper from acidified copper sulfate solution. Variables studied were sphere diameter, bed height, and physical properties of the solution. The mass transfer coefficient was found to decrease slightly with increasing bed height and independent of sphere diameter. The data were correlated for the conditions 5 x 10(6) < Sc.Gr < 5.4 x 10(8) by the equation Sh = 0.28(Sc.Gr)(0.32). A comparison between the present and some previous data at other packing geometries shows that the rate of natural convection mass transfer at a bed of spheres is higher than that at beds of Raschig rings and at cylinders. The importance of the present results for the design and operation of catalytic and electrochemical reactors used to conduct diffusion controlled liquid-solid reactions is highlighted.