The rates of mass transfer at a vertical array of closely spaced horizontal tubes were measured by the limiting-current technique under single-phase flow, gas sparging and two-phase flow. The single-phase flow data were correlated by the equation: Sh = 0.75 Sc-0.33 Re-0.59. The gas sparging data with no net solution flow were correlated by the equation: J = 0.3 1 (Re-g.Fr)(-0.22). For two-phase flow, the gas flow was found to enhance the rate of array mass transfer by a factor ranging from 1.25 to 5.25, depending on Re-g and Re. The enhancement ratio increases with decreasing Re and increasing Re-g. For Re >= 2500, the rate of mass transfer approaches the value of single-phase flow, regardless of the value of Re-g, which ranged from 7 to 41. The importance of the present geometry in building electro-chemical and catalytic reactors, where exothermic liquid-solid diffusion-controlled reactions take place, is highlighted. The present geometry offers the advantage that the outer surface acts as a turbulence promoter while the inner surface acts as a heat exchanger.