Experiments were performed at ambient temperature and pressure in a 127-mm inner diameter column with air, 1.2 and 5-mm spherical glass beads and various aqueous solutions containing surface-active agents. Perforated plates were employed as the gas and liquid distributors. Gas and liquid velocities were chosen to span the dispersed and coalesced bubble flow regimes. The dynamic surface tension behaviour of a specific liquid solution correlated to its ability to form stable foam. However, the magnitude of the impact of surface-active agents on phase holdups depended primarily on the operating conditions. The presence of surface-active agents increased the gas holdups in a bubble column by an average of 41%. In a three-phase fluidized bed, surface-active agents increased the gas holdups by an average of 37% while the liquid and solids holdups were reduced by an average of 6.2 and 4.5%, respectively. There is still no reliable means to estimate the gas holdups of three-phase fluidized beds subject to foaming. More work is required to quantify the interaction effects of surface-active agents and operating conditions on the hydrodynamic features of multiphase reactors. (c) 2006 Elsevier B.V. All rights reserved.