The Pressure-Volume-Temperature (PVT) dependencies of polystyrene-based clay-containing nanocomposites (CPNC) were determined in the glassy and molten state. The PVT data in the melt were fitted to the Simha-Somcynsky (S-S) lattice-hole equation-of-state (eos), yielding the free volume quantity, h = h(T, P), and the characteristic reducing parameters, P*, V*, T*. The data within the glassy region were interpreted considering that the latter parameters are valid in the whole range of independent variables, than calculating h = h(T, P) from the experimental values of V = V(T, P). Next, the frozen free volume fraction in the glass was computed as FF = FF(P). In the molten state the maximum reduction of free volume was observed at w(solid) approximate to 3.6-wt % clay, amount sufficient to adsorb all PS into solidified layer around organoclay stacks. In the vitreous state FF increased with clay content from 0.6 to 1.6-this is the first time FF > 1 has been observed. The highest value was determined for CPNC with the highest clay content, w = 17.1 wt %, thus well above w(solid). The derivative properties, compressibility, kappa, and the thermal expansion coefficient, alpha, depend on T, P, and w. Plots of kappa versus T indicate the presence of two secondary transitions, one at T-beta/T-g approximate to 0.9 +/- 0.1 and other at T-T/T-g = 1.2 +/-0.1. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2504-2518, 2008