Gas holdup in bubble columns containing structured packing was determined for varying liquid phase viscosity and different construction materials of the packing. Three columns, containing packing made from smooth nickel plate, perforated nickel plate and plastic sheets, respectively, were tested. The gas holdup in a column with no internals was also measured for comparison purposes. The effect of viscosity on gas holdup was studied by using water, aqueous glycerol and aqueous CMC solutions as the liquid phase; some of the solutions showed non-Newtonian behavior. The experimental results of gas void fraction were correlated in terms of superficial gas velocity and liquid viscosity through a simple homographic expression. The geometric characteristics of the structured packings were found to influence gas holdup, thus yielding slightly different equations for each structured packing. This effect is discussed in detail. Nevertheless, for engineering purposes a single equation representing all the data is also proposed, which permits the prediction of gas holdup in structured packing with sufficient accuracy. (C) 2004 Elsevier Ltd. All rights reserved.