The effect of gas sparging on the rate of mass transfer at horizontal single screen and stacks of closely packed horizontal screens was studied by an electrochemical technique which involved measuring the limiting current of the cathodic reduction of potassium ferricyanide. Variables studied were: distance between the sparger and the screen, screen characteristics, e.g., mesh number and wire diameter, physical properties of the solution and superficial gas velocity. Screen characteristics were found to have little effect on the rate of mass transfer. The mass transfer measurement at beds of closely packed screens revealed that the mass transfer coefficient decreased below the single screen value with increasing the number of screens per bed. Comparison of the present data with previous results showed that screens produce higher rates of mass transfer than other geometries under otherwise the same conditions. The importance of the present work to the design and operation of catalytic and electrochemical reactors was highlighted.