An experimental study of the liquid holdup and the liquid holdup axial profile in a square section column with structured packing is carried out. Both cocurrent and countercurrent operations are examined. A conductivity technique to estimate liquid holdups is proposed and calibrated against values measured by the drainage method. Liquid holdups estimated by this technique follow the same trends as those previously found by other methods. Axial profiles of liquid holdup in the cocurrent and countercurrent operation are illustrated for liquid velocities below the loading point, and for solutions of different viscosity and foaming character. Variations between operation modes are larger for the foaming liquid than for the other liquid solutions employed. (c) 2005 Elsevier Ltd. All rights reserved.