The possibility of overcoming oxygen limitations in membrane-attached biofilms has been investigated by using nitrate as an electron acceptor instead of oxygen in an extractive membrane bioreactor (EMB) degrading toluene. The effect of nitrate concentration on toluene flux, the effective diffusivity in the biofilm and the biofilm activity has been investigated. A counter-diffusion-reaction model is also presented, describing the pollutant flux versus biofilm thickness. The toluene flux decreased with increasing biofilm thickness under excess nitrate concentrations, similar to the experiment with low nitrate. Mathematical modelling indicated that this was either due to decreasing activity, and/or different diffusivities in the biofilm. The effective diffusivity was investigated by using an inert tracer molecule. It remained constant for biofilm thicknesses up to 1.8mm, with a value twice that in water. The biofilm activity was investigated by inactivating a mature biofilm using sodium azide. The toluene flux remained the same before and after the addition of sodium azide, suggesting that the activity in the biofilm is very low. We conclude that the decreasing toluene flux with increasing biofilm thickness is due to the diffusional resistance of the inactive biofilm.