The theory of diffusion and reaction has been applied to describe mass transfer and reaction phenomena in membrane-attached biofilms (MABs) growing in extractive membrane bioreactors (EMBs), and to establish the rate-limiting mechanisms in these systems. The model formulated accounts for substrate counter-diffusion and two-limiting-substrate reaction within the biofilm. Model simulations are compared to experimental data obtained in a lab-scale EMB and a simple case study is considered to show how MABs affect performance of EMBs. It is found that the organic substrate flux across the membrane is strongly affected by MABs, which constitute an additional resistance to mass transfer and in most cases reduce the flux across the membrane. As a result of the investigation, it is concluded that the decrease of flux in the presence of MABs, observed experimentally and predicted theoretically, is dominated by the resistance to organic substrate transfer caused by the biofilm.