A complicated three-dimensional (3D) transient computational fluid dynamics (CFD) model was established to simulate the biotreatment of toluene waste gas in a lab scale airlift loop reactor (ALR), with multiphase flow, mass and momentum transfer and bioreaction coupled together. Multiphase flow and bioreaction were bridged by mass transfer. The model simulations were verified by the local averaged axial liquid velocities, the toluene removing efficiency (RE) and the local transient liquid phase dissolved toluene (DT). The predictions of the system shock resistance and the local transient dispersions of variables including the hydrodynamic parameters and the component concentrations were given. Furthermore, the relationship between mass transfer and bioreaction in ALR and in the draft tube were discussed quantitatively to determine the rate-limiting step. Toluene bioreaction and oxygen mass transfer are the limiting steps in the whole ALR; toluene and oxygen bioreactions are the rate-limiting steps in the draft tube. The conclusions disclose that rate-limiting step for the biotreatment process is complex and multivariate with the components and the regions in ALR. (C) 2010 Elsevier B.V. All rights reserved.