A laboratory scale membrane aeration bioreactor (MABR) with a void volume of 1.351 was tested for its ability to treat synthetic wastewater. The MABR process couples aerobic biological wastewater treatment with the bubbleless mass transfer of pure oxygen through hollow fibre membranes. The biofilm that was formed on the surface of the hollow fibres utilised oxygen transferred from the fibre lumen for the degradation of organics transferred from the synthetic wastewater flowing over the biofilm. When operated as a completely mixed reactor at an organic loading rate of 24.5 kg chemical oxygen demand (COD)/m(3)/day and a hydraulic retention time of 34 min, an 89% COD removal efficiency was achieved. During plug flow operation, 86% COD removal efficiency was obtained at an organic loading rate of 5.8 kg COD/m(3)/day and a hydraulic retention time of 47 min. Regular membrane cleaning to control biomass growth was necessary to prevent channeling. The ability of the reactor to operate under plug flow conditions demonstrated that the biofilm plays a significant role in oxygen mass transfer.