Fluid dynamic gauging was used to investigate the cohesive strength of the membrane fouling layer formed during cross-flow microfiltration of microcrystalline cellulose. Fouling behaviour was compared at two pH levels (i.e. different surface charges of the particles and membranes) with two membranes (i.e. regenerated cellulose and polyethersulphone). It was found that a suspension at low pH, where the surface charge of the particles is close to zero, resulted in thicker and stronger surface fouling layers (668 +/- 66 mu m thick at a shear stress of 36 Pa for the regenerated cellulose membrane). The permeate flux was reduced by 62% during the first 1000 s. For close-to-neutral pH, where the particles are negatively charged, the fouling layers were thinner and less resistant to shear stress (290 +/- 77 mu m thick at a shear stress of 36 Pa) and the decline of the flux was faster: a 90% decrease was recorded during the initial 1000 s. The differences in flux decline behaviour suggest a more pronounced blocking of the pore openings for the membranes at the higher pH. Similar fouling behaviour was observed for the two membranes. An atomic force microscope equipped with a colloid probe was used to evaluate particle/particle and particle/membrane interactions. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.