In this paper a CFD model for a bubble column reactor undergoing a first order reaction A --> B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter D-T = 1 m and height HT = 5 in. The incoming gas stream contains inerts, varying in proportion from 10% to 90%. Three-dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity U-G = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts., The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.