The gas holdup and liquid axial dispersion coefficient are measured in two semibatch packed bubble columns, 0.154 and 0.200m diameter for an air-water system, at atmospheric conditions. It is observed that the one-dimensional dispersion model does not give an accurate description of the tracer concentration profiles from a pulse injection. This is due to convective liquid flows inside the bed and poor radial mixing. The liquid circulation comprises an upward flow in the column core and a downward flow along the wall. Extension of the dispersion model to account for the convective recirculation is discussed. It is also seen that the radial mixing increases considerably in the pulsation flow regime and as a result of this; the dispersion coefficient reduces suddenly at the transition point between the bubble Row and the pulsation Row regime. (C) 2001 Elsevier Science B.V. All rights reserved.