In membrane bioreactor (MBR) systems, several fouling mechanisms occur; e.g. pore blocking, gel formation, biofilm growth and cake formation. This study focuses on the cake layer which is easily removed e.g. by relaxation and the inner gel layer that is not removed by relaxation. The cake layers are compressible and significantly influence MBR operation, especially for short term filtrations. Over longer filtrations, an inner gel layer is formed. The gel layer consists of soluble microbial products (SNIP). To investigate whether this structure exhibits a pressure dependent filtration resistance, the compressibility of the fouling layers was studied by a TMP step procedure. The data from the experiments show that the gel layers are compressible. Further, there is a pressure in the range of 4.9-7.9 kPa, where a compressive yield stress was observed, below which the gel is incompressible and above which the gel is compressible. The compression was reversible, both for the cake layer and for the gel layer. This shows that the fouling layers formed in MBRs are highly compressible which should be accounted for during both short and long term operation. (C) 2014 Elsevier B.V. All rights reserved.