Experiments were designed to characterize the oxygen transfer across a micro-porous, flat-sheet membrane with and without a nitrifying membrane-aeratedbiofilm(MAB). Clean membrane oxygen transfer was quantified via average mass transfer coefficients and local fluxes calculated from profiles of dissolved oxygen (DO) along the membrane length. Stoichiometric calculations and DO profiles were used to characterize oxygen transfer with a nitrifying MAB. Comparison of the local fluxes with clean and biofilm coated membranes revealed that oxygen transfer was decreased in upstream sections of the membrane during cultivation of an MAB due to reduced advection and/or turbulence near the membrane surface. By contrast, oxygen transfer was increased in downstream sections of the membrane via bacterial respiration. Oxygen fluxes generally decreased in the downstream direction with biofilm coated membranes; however, variability in biofilm structure and membrane coverage altered this trend on several occasions. Furthermore, biofilm structures were observed to form via sloughing and settling of biomass at the membrane Surface. This suggests that reactor configuration may have a significant influence on oxygen transfer rates in membrane-aerated biofilm reactors (MABR). (c) 2005 Elsevier B.V. All rights reserved.