Several anaerobic membrane bioreactors ( AnMBR) were operated, under various conditions, applying different reactor configurations. Applicable fluxes were strongly determined by the physical properties of the sludge present in the reactors. Results show that particle size is a key determining factor for the attainable fluxes. Under thermophilic conditions, small sludge particle size was observed, resulting in low critical fluxes reaching 6 - 7 L/ m(2)h for the submerged configuration and acidified substrate. In contrast, under mesophilic conditions critical fluxes of 20 L/ m2h were obtained. The acidification level also showed a strong effect. Under thermophilic conditions, the presence of a significant fraction of non- acidified organic matter induced the growth of suspended acidogenic biomass that seriously affected the applicable fluxes, both in submerged and side- stream configurations. Under all conditions tested cake formation showed to be the limiting factor determining the applicable fluxes. Only low levels of irreversible fouling were observed. Due to technical and economical considerations, most interesting perspectives for the application of AnMBR are expected with the treatment of high- strength particulate wastewaters, and with extreme wastewaters characterised by high temperature, salinity, etc.