Five samples of fine particle size calcium carbonate, typically used as excipient or as active pharma ingredient (API): a powder, two granulated samples (compressible precursor) and two tablet samples, were analysed to establish the pore size distribution within their packed structures. Three different measurement techniques have been used to characterise the samples, mercury porosimetry, hexadecane imbibition and hexadecane permeability. The structures have also been modelled using the Pore-Cor network simulator. The effective pore size distribution and its origin are used to demonstrate the impact of particle size distribution of the starting material powder in order to create denser tablets and hence smaller, more easily swallowable tablet forms. The application of the network simulator illustrates the importance of taking account of pore shielding when describing potential solutions to liquid uptake and dissolution dynamics of the end product in drug delivery. The work shows that ground calcium carbonate, with its close to log-normal particle size distribution, provides an improved packing, in comparison to typical monosize precipitated calcium carbonate, under tablet-forming conditions with well-defined absorption and permeation properties. (c) 2006 Elsevier B.V. All rights reserved.