Quite apart from its effect on the transfer area, a knowledge of frothing in distillation is of importance in deciding design features such as tray spacing and down-comer size. A two tray sieve plate column was therefore built to work with the systems air/water, air/glycerine-water and air/glycerine-water-sodium lauryl sulphate. The downcomers were external to the column and transparent so that visual observations can be made. The downcomer flow patterns and the effect of air rate, liquid rate, surface tension, and downcomer size on the frothing and aeration in the downcomer and on the plate have been investigated. It has been demonstrated that present methods of downcomer design are inadequate, and the classical concept of flooding viz. "if the froth in the downcomer reaches the top of the weir the column would flood," is not true. New criteria for flooding and downcomer design have been suggested.