The study investigated the stability of the simultaneous nitrification and denitrification process in membrane bioreactors with specific emphasis on mass transfer limitations for oxygen diffusion. It mainly focused on the effect of biomass (MLSS) concentration on the relative magnitude of mass transfer coefficients. For this purpose, a functional relationship was derived between MLSS concentrations in membrane bioreactor and the oxygen half saturation parameters based on available experimental results. Model simulation using the generated data indicated that full nitrogen removal could be achieved in MBR systems operated at different MISS levels with the provision of selecting optimal DO set-points corresponding to each operating conditions. The required optimal DO set-points increased with higher biomass concentrations due to higher mass transfer limitation and they remained operative in a wider DO range. Elevated MLSS levels required higher aeration energy and a relatively less robust DO set-point control approach was sufficient. The simulation results also indicated that MBR operation with a level of around 12,000-14,000 mg MLSS L(-1) provided an optimal compromise between reducing the reactor footprint and minimizing mass transfer limitation for effective nitrogen removal. (C) 2010 Elsevier B.V. All rights reserved.