This article focuses on the physical modeling and numerical simulation of mass transfer in two-phase bubbly flow in an airlift internal loop reactor. The objective of this article is to show the ability of computational fluid dynamics (CFD) to correctly simulate mass transfer in such a bubbly reactor. The modeling of two-phase bubbly flow is based on the so called two-fluid model derived from Reynolds-averaged Navier-Stokes equations in two-phase flow. From the hydrodynamic perspective, the flow is steady state. Given the steady-state distributions of phases, interfacial area, and velocity field in the whole volume of the airlift, mass transfer is computed and the evolution of oxygen concentration in the two phases is predicted. Numerical simulations are discussed after comparison to experimental data. The simulations are validated in terms of oxygen concentration in the liquid vs. time. Then, different points are discussed, in particular, the perfectly mixed reactor assumption in the liquid phase and the spatial and temporal heterogeneity of oxygen concentration in the gas arising from oxygen impoverishment in bubbles in the downcomer. This leads to heterogeneity of the transfer driving force between the gas and the liquid. Bubble age or residence time in the airlift loop reactor has been calculated to show the weak renewal of oxygen in bubbles in the downcomer. This discussion generates questions on the estimation of a global mass transfer coefficient from experiments in such a heterogeneous airlift reactor. (c) 2007 American Institute of Chemical Engineers.