Measurements of gas holdup were made in bubble columns of 0.1, 0.19 and 0.38 m diameter with air as gas phase and Tellus oil as the liquid phase. The gas holdup was found to decrease with increasing column diameter. The reason for this scale dependence is because the strength of the liquid circulations increases with increasing scale. Such circulations accelerate the bubbles travelling upwards in the central core. Computational fluid dynamics (CFD) simulations were carried out using the Eulerian description for both the gas and the liquid phases in order to model the scale dependence of the hydrodynamics. Interactions between the bubbles and the liquid are taken into account in terms of a momentum exchange, or drag, coefficient; this coefficient was estimated from the measured gas holdup data at low superficial gas velocities. The turbulence in the liquid phase is described using the k-epsilon model. The CFD simulation results are in good agreement with the experimental results for all three columns and provide a reliable strategy for scaling up to reactors of commercial size.