Data are reported on particulate and process variables which influence the formation of compressible filter cakes. Variables such as surface charge, particle size and size distribution, filtration pressure and solids concentration have been investigated. To obtain a greater insight into the filter cake behaviour, an electrical impedance imaging technique for determining the in-situ solids concentration in solid/liquid mixtures has been used. Pressure leaf filtration tests of wellcharacterised solids dispersed in aqueous media have been performed over a range of conditions to investigate the characteristics of compressibility. The majority of the results from these tests are shown through solids concentration profiles as functions of suspension/cake height and time, and specific cake resistance and porosity as functions of pressure. The dependence of the compressibility on surface charge and other variables has been found. Comparisons have been made with the Tiller/Shirato ‘modern’ filtration theory and the difficulties highlighted.