Membrane fouling is widely reported to be dependent on membrane/foulant interactions (i.e.: hydrophilicity/hydrophobicity, electrokinetics interactions) and on the interactions between foulant and compounds already deposited at the membrane (fouling layer). Those complex interactions could modify and limit mass transfer phenomena across the membrane. Consequently, knowing the electrokinetic properties (EP) of the fouling layer is a key point for understanding fouling but there is a need of new methods to characterise the EP of fouling layers. Indeed, existing methods for measuring the EP of fouled membranes are scarce in the literature especially for an in line characterisation (i.e. during cake layer build-up) or are not applicable to the case of complex fouling layers (such as compressible fouling layers). This study introduces a new measurement method at constant pressure during the filtration to allow a link between electrical potential values and filtration operating parameters (such as deposited mass on membrane surface or transmembrane pressure). The results given in this paper are dedicated to EP of a microfiltration polysulfone membrane (pore size of about 0.1 mu m) being fouled by negatively charged latex particle suspensions (particles diameters of 200 nm. zeta potential: -27 mV). Experimental results show that the zeta potential value of a clean and fouled membrane obtained by the new protocol are the same as the classical streaming potential measurement (i.e. the ex situ protocol with pressure steps currently described in literature). This validates the new protocol that can be exploited during the growth of cake layer on membrane surface. Results demonstrate that the influence of the transmembrane pressure and particles deposited mass on the electrical potential variations and on the fouling rate are similar and so doing that the electrical potential variation during cake layer formation could be used as a new fouling indicator. The sign cif the electrical potential variation can give qualitative information of electrostatic charges of the cake layer being formed. Finally, during cake layer formation, all electrical potential variation against permeate flux can be plotted in a triangle limited by classical streaming potential values. (C) 2010 Elsevier B.V. All rights reserved.