Rates of mass transfer were studied at a vertical array of closely packed screens under single and two phase (gas-liquid) flow by measuring the limiting current for the cathodic reduction of ferricyanide ions. Variables studied were screen characteristics (mesh number and wire diameter), physical properties of the solution, solution flow rate, gas flow rate and the effect of surface active agents. The single phase data were correlated by the equation: J = 0.52 Re-L(-0.55) while the two phase data were correlated by the equations: Sh=0.87 Sc-0.33 Re-L(0.35) Re-g(0.12) for the conditions 10 < Re < 125 and 1.4 < Re-g < 77; and Sh=0.62 (ScReLReg0.25)-Re-0.33-Re-0.11 for the conditions 1.1 < Re-L < 22 and 1.4 < Re-g < 77. The presence of surfactant was found to reduce the rate of mass transfer in both single phase and two phase flow, the percentage reduction being higher in the case of single phase flow.