The structure of the free volume and its temperature dependence between 25 and 200degreesC of copolymers of styrene with maleic anhydride, SMA (0-35 mol% MA), is studied by pressure-volume-temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this first part of the work, PVT data are reported which were analysed with the Simha-Somcynsky equation of state to estimate the volume fraction of holes, h, which constitute the excess free volume. The temperature and pressure dependence of the specific volume V, the specific occupied and free volume, V-occ = (1 - h)V and V-f = hV, and the corresponding isobaric expansivities and isothermal compressibilities for both the rubbery and glassy state are estimated. We obtained the unexpected results that (i) the occupied volume changes its coefficient of thermal expansion at T-g from alpha(occ,g) approximate to 0.5alpha(g) approximate to 1 X 10(-4) K-1 below T-g to almost zero (approximate to 0.2 X 10(-4) K-1) above T-g and (ii) the isothermal compressibility of the occupied volume at zero pressure below T-g is rather high, K-occ approximate to 2.5 X 10(-4) MPa-1, and decreases only slightly at T-g to about 2 X 10(-4) MPa-1 above T-g. The variation of total, occupied, and free volume parameters with the composition of the SMA copolymers is discussed. (C) 2004 Elsevier Ltd. All rights reserved.