Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper. a computational fluid dynamics (CFD) model is developed to describe the hydrodynamics, and mass transfer, of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: "small" and "large" bubbles. For the air-water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag. coefficients. which differ for the "small" and "large" bubbles. The turbulence in the liquid phase is described using the k-epsilon model. For bubble columns operating with the air-water system, CFD simulations have been carried out for superficial gas velocities. U. in the range 0-0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of I homogeneous and heterogeneous flow regimes, and of regime transition on the gas holdup and mass transfer. By comparing the simulations with measured experimental data, it is concluded that mass transfer from the large bubble population is significantly enhanced due to frequent coalescence and break-up into smaller bubbles. The CFD simulations also underline the strong influence of column diameter on hydrodynamics and mass transfer. (C) 2003 Elsevier Science B.V. All rights reserved.