Polycyanurate networks, based on the dicyanate of bisphenol A monomer, were synthesized with different extents of reaction ranging from 80% to 100%. The effects of conversion, as measured by d.s.c., on the volumetric properties in the glassy state were investigated. The variations in density, packing density and coefficient of volume expansion were evaluated and then compared with the free-volume holes deduced from positron annihilation spectroscopy lifetime and to moisture absorption. Amongst the various reasons that could explain the large density variation, steric hindrance is the dominant factor: the bulky triazine crosslinks are responsible for the increase in free-volume holes which occurs as the conversion increases and controls the water uptake. (C) 1997 Elsevier Science Ltd.