Cake layer formation in anaerobic gas-sparged submerged membrane bioreactors was studied using the critical flux concept, at 30 and 55 °C. The impact of biomass concentration, from 25 to 50 g TSS/L, and superficial gas velocity, up to 70 m/h, of over cake layer formation was studied, using response surface methodology. Under mesophilic conditions, assessed critical flux values were in the range 5-21 L/m2 h, while a first order relation between critical flux and both biomass concentration and gas flow rate was observed. Within studied range of experimentation, at 30 °C, effect of biomass concentration on critical flux was three times higher than the effect of gas superficial velocity. Under thermophilic conditions, critical flux values were in the range 16-23 L/m2 h, even though gas superficial velocities were about 50% lower than in the mesophilic reactor. At 55 °C, both biomass concentration and gas superficial velocity presented a value, below and over which no further effect on critical flux was observed. On a short-term basis (minutes or hours), cake layer formation was mainly reversible. However, cake layer formation proceeds fast once critical flux has been exceeded, restricting feasible operational flux to the range of the critical flux. Better performance of thermophilic reactor opens interesting perspectives for the application of membrane anaerobic bioreactors at high temperatures, condition where biomass aggregation is difficult to achieve.