The aim of this project was to enhance the current state of knowledge of the void structure of paper, and pigments used to coat paper. The porosities and pore size distributions of consolidated pigments were measured using mercury porosimetry. Prior to this work, mercury porosimetry was rarely used in this field due to problems associated with the conventional interpretation of mercury porosimetry data. These problems were examined and two limitations of mercury porosimetry were addressed. Firstly, the shrinkage of compressible samples causes an apparent increase in void volume and secondly, large void spaces shielded by smaller ones are not intruded until anomalously high applied pressures of mercury are reached. The first limitation was overcome by means of a new correction procedure which, uniquely, also allows the measurement of the bulk modulus of the continuous solid phase of a porous sample. Shielding effects have been taken into account by means of a software package known as Pore-Cor, which generates a three dimensional structure which has both a mercury intrusion curve and porosity in close agreement with experiment. It has also been possible to calculate the permeabilities and tortuosity of the simulated structure and this provides realistic and useful values, which may not be measured experimentally. Mercury porosimetry and a range of specialised absorption techniques, including liquid porosimetry, were used to characterise the porous structures of a highly filled paper which had been calendered using a range of different conditions. A unique feature of this work is that for the first time it has been shown that two porosimetric techniques which measure overlapping pore size distributions may be combined to give a better indication of the total pore size distribution.