We demonstrate the use of a quartz crystal viscometer to determine the viscosity of solutions of polystyrene in limonene in the presence of high pressure carbon dioxide. These measurements were determined up to 50 bar in the range of temperature from 20 to 40 degrees C and at 0.025, 0.05, 0.1, 0.2, 0.3 and 0.4 g PS per ml of limonene. The selected variables of study (pressure, temperature and concentration) were statistically significant over the range of viscosity studied. The viscosities of the solutions at all loadings were found to decrease with increasing temperature and pressure while the plastising effect of CO2 prevailing over the hydrostatic pressure applied by the gas. The flow activation energies of the system were obtained from an Arrhenius fitting of the experimental data reaching a maximum of 40 kj/mol. An empirical model correlating the viscosity of the mixtures to pressure and temperature was developed in order to interpolate and extrapolate in the study region or in a range close to the working limits. Finally, the viscosity data were used to determine the solvent quality and to quantify the demixing points of the mixtures. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.