Constant rate expression of fluid from deformable particulate solids was studied at pressing speeds of 2 and 5 cm/min. Spent drip-grind and high-temperature extracted coffee grounds were used as test materials. The cell walls of the latter were partly hydrolyzed as result of the extraction process. Comparison of the results obtained with these two types of grounds and with fluid viscosities of 1 and 15 CP made possible a qualitative analysis of the effect of cell wall permeability on the cake porosity. A technique was developed to measure the cake porosity as a function of the cake specific volume. The particles were contained in a cylindrical expression cell, and the extraparticle void space was filled with a tracer solution of known concentration. As the particles were compressed, exudation of intracellular fluid begun and the tracer solution was diluted. The extend of dilution was monitored at the outflow and measured by spectrophotometric techniques. Initial compation resulted mainly in a reduction of the extraparticle void volume, followed by exudation and particle collapse. In the case of drip-grind coffee grounds, exudation started at a cake specific volume of 4.2 cc/g. In the case of high-temperature extracted coffee grounds, exudation begun at an earlier stage of compaction (higher specific volume). The cake porosities were calculated from a mass balance on the tracer. With drip-grind grounds as the test material, cake porosities were found to decrease at first with compaction, to a value of 0.3 at a cake specific volume of 2.5 cc/g, and increased to 0.37 with further compaction. When the fluid viscosity was increased by soaking the grounds in a 50% by weight sucrose solution, this increase in porosity was not observed. With high-temperature extracted grounds as the test material, a minimum porosity of 0.3 was also observed, but at a cake specific volume of 3.7 cc/g. In this case further compaction also resulted in an increase in porosity to 0.37 at maximum compaction. There was no significant difference between results obtained at pressing speeds of 2 and 5 cm/min. The cake porosity data were used in the Kozeny-Carman equation to predict the fluid pressure drop across the cake. There was lack of agreement between the calculated pressure drops and direct measurements obtained in parallel investigations. This disagreement was mainly the result of a change in particle sphericity with compaction. The exudation pressure drop was calculated from the rate of intracellular fluid exudation. It was assumed that during compaction there is no cell rupture, and that exudation takes place through the coffee cell pores on the surface of the particle, and that the flow can be satisfactorily described by Poiseuille’s law. This model of exudation was found to be inappropriate because considerable cell rupture occurs during cake compaction.