The (p, V, T) behaviour and the miscibility of polysulfone in binary fluid mixtures of tetrahydrofuran (THF) and carbon dioxide (CO2) have been investigated in the temperature range from (300 to 425) K at pressures up to 70 MPa. Densities of polysulfone solutions (mass fraction = 0 to 0.496) in a solvent mixture of THF and CO2 with a mass fraction of CO2 of 0.1 were determined as a function of pressure from the homogeneous one-phase region through the phase separation point into the two-phase region at selected temperatures. The densities at the demixing pressures at these temperatures were also determined. No significant changes in density were found across the phase boundary, indicating that coexisting phases must have similar densities, which is often the case with (liquid+liquid) phase separation in high-pressure systems. The variations of density or specific volume with polymer concentration have also been investigated at selected temperatures and pressures. The data show unique features which are helpful in understanding the phase behaviour of this system which shows multiple miscibility windows. The isothermal compressibility of the polymer solutions as determined from the density data does not show any significant dependence on polymer concentrations. The compressibilities display the expected decrease with pressure and increase with temperature. (C) 2003 Published by Elsevier Science Ltd.