Pore volume of paper sheets as measured by mercury intrusion porosimetry can provide an alternative structural description to solid phase-based measures such as density and may be used to examine sheet structure. We measured sheet densities and specific pore volume, i.e. the pore volume per unit sheet mass, for sheets made from whole pulp and the long fibre fraction of five different pulps. The coefficient of variation was 50% and 28% for specific pore volume and density of different pulps, respectively. Pore volume was more sensitive to structural differences than density. A theoretical measure of specific pore volume of the long fibre fraction (R48) was derived from the Interactive Multi-Planar Model (IMPM) of sheet structure. For the whole pulp we assumed that the shorter material (P48) filled voids and thus diminished the specific pore volume of the long fibre fraction (R48). Model predictions of specific pore volume agreed well with mercury porosimeter determinations for most of the samples. The effect of P48 fraction on sheet porosity was greater for newsprint than for hardwood kraft pulps.