The concept of critical flux (J(c)) has been recognised as a convenient parameter for characterising fouling in membrane bioreactors (MBR), although it has recently been shown that membrane fouling takes place in this specific process under sub-critical operating conditions. It is also generally understood that the permeability of membranes used for biomass separation membrane bioreactors is influenced by the method and rate of aeration, the nature and concentration of the sludge and the membrane characteristics. In this study, J(c) values have been determined using the flux-step method for various operating conditions of membrane pore and lumen size, mixed liquor suspended solids (MLSS) concentration and aeration rates, and sub-critical operation has also been investigated. Results from factorial experimental design analysis have revealed that (1) the impact of membrane pore size on J(c) is observed only at low pore size and/or low MLSS levels, (2) no significant difference is apparent for a shift from 4 to 8 gMLSS 1(-1), but a significant increase of J(c) arises for an MLSS increase to 12 g 1(-1), and (3) the MLSS effect on J(c) was generally around double that of the aeration effect. The calculation of mean sub-critical values for a range of pressure-related critical parameters revealed reduced short-term fouling at larger-pore sizes, but that this may be coupled with internal membrane fouling. (C) 2003 Elsevier Science B.V. All rights reserved.