A steady-state mathematical model has been developed to predict axial-concentration profiles of a pollutant in an extractive-membrane bioreactor (EMB). Typically, the previous models describe the pollutant concentration profiles in the membrane-attached biofilms in a direction perpendicular to the membrane. In contrast, the model presented in this work describes not only the radial profiles, but also the axial profiles along the membrane length. Biofilms of Xanthobacter autotrophicus GJ10 were grown on the surface of silicone rubber tubes. A diffusion-reaction model was employed to describe the diffusion and reaction in the biofilm in the radial direction. Membrane tubes were modelled as a series of mixed tanks to allow the prediction of axial concentrations. The model predictions were verified by experimental data from a range of operating conditions. These included different dissolved oxygen concentrations in biomedium and different wastewater flowrates. Finally, the rate-limiting step in the reactor was determined to be the mass-transfer resistance of the pollutant in the biofilm.