Removal of liquid from filter cakes can be accomplished by mechanical or hydraulic methods after cake formation is The local porosity in porous beds (Tiller and Cooper, 1962) is a complete. This paper deals with the latter procedure. function of hydraulic pressure distribution and cake compressibility. Calculation of average porosity requires an integration of local values as determined by liquid flow patterns. As most compressions of filter cakes are irreversible, the local porosity is a function of the maximum effective pressure (frictional pressure loss) reached during cyclical operations. Reversal of flow through a cake develops radically changed pressure distributions which can be utilized to reduce local porosities. Analytical expressions are presented for reduction of average porosity brought about by reversing flow of liquid in plate-and-frame and recessed plate filter presses.