Extrusion foaming of molten polystyrene (PS) with three physical foaming agents (PFAs), carbon dioxide (CO2), 1,1,1,2-tetrafluoroethane, and 1-chloro-1,1-difluoroethane, is considered. The concentration of injected PFA was W = 0-5 wt % for CO2 and W = 0-15 wt % for the other agents. The aim of this work is to connect flow and equation of state (EOS) properties under the temperature and pressure conditions encountered during extrusion foaming. The constant-stress viscosity eta at sigma (12) congruent to 40 kPa was measured online at temperatures T congruent to 110-210 degreesC and pressures P congruent to 5-13 MPa. The EOSs of PS and PFA are analyzed in terms of the Simha-Somcynsky lattice-hole theory. The hole fraction, h = h(T,P), is extracted from the analysis of the experimental pressure-volume-temperature data, and a conventional free-volume fraction is also obtained and related to h. Next, these functions are related to the constant-stress viscosity of PS/PFA mixtures in terms of alternative mixture rules. The T, P and composition dependencies of the system viscosity can be satisfactorily expressed in terms of volume-average hole fractions of the two constituents. An analysis of Newtonian viscosities of PS/PFA systems measured by Kwag et al. [Kwag, C., Manke, C. W., and Gulari, E., J Polym Sci Part B: Polym Phys, 1999, 37, 2771] under steady-state conditions results in a satisfactory agreement with the developed procedure. (C) 2000 John Wiley & Sons, Inc.