A common challenge during membrane filtration is cake fouling, whereby the build-up of material on the membrane surface reduces the permeate flux. Such fouling layers can also alter the selectivity of the separation. In this study, fluid dynamic gauging (FDG) is used in situ to investigate the cake fouling formed during cross-flow filtration of a model material: softwood Kraft lignin. FDG was used to estimate (i) the thickness of the cake layers (in the gm scale) and (ii) the local cohesive strength at different depths in the cake layer. Fouling layers formed at different transmembrane pressure (TMP) values were investigated. The estimated thickness of the cake layers increased with increasing TMP. However, it was difficult to capture the full cake thickness for the more loosely formed cakes layers. An increase in the cohesive strength of the cake was found to occur with increasing TMP values. (C) 2017 Elsevier B.V. All rights reserved.