The structure of a fouling cake layer building up during membrane filtration can have a decisive influence on the performance of the membrane process. Hence a comprehensive description of the cake layers structure is mandatory. In this work we report a new methodology of simultaneously recording hydraulic resistance and electrical impedance during a membrane filtration experiment for flat sheet as well as hollow fiber membranes. An equivalent circuit model is derived to describe the development of the measured impedance during filtration. In situ e.g filter cake height and filter cake porosity as a function of time can be extracted from the experimental data for flat sheet as well as for hollow fiber membranes. The extracted data support traditional filtration theories for dead-end filtration on flat sheet membranes predicting an increase of cake layer porosity with proceeding filtration. However, the data obtained from inside-out filtration using a hollow fiber membrane reveal an apparent decrease of cake layer porosity during filtration. So the decreasing deposition area for the particles in inside -out filtration apparently results in a higher packing density of the particles and thus lower cake layer porosity. The electrical impedance data give an additional correlation allowing us to characterize the fouling cake layer structure or composition. (C) 2016 Elsevier B.V. All rights reserved.