High-pressure viscosity of polystyrene (M-w = 50,000, M-w/M-n less than or equal to 1.06) solutions in toluene and in mixtures of toluene + carbon dioxide was measured using a falling cylinder-type viscometer at 320, 340, and 360 K, and up to 35 MPa. Solutions with polystyrene concentrations of 3, 5, and 7 wt % were investigated. Carbon dioxide levels in the range from 0 to 14.7 wt % were evaluated. Viscosity was observed to vary linearly with pressure at the temperatures and polymer concentrations investigated. Viscosity of the polymer solutions decreased as the concentration of carbon dioxide in the mixture was increased. The largest viscosity reduction was observed at the lowest temperature and at the highest concentration of polymer. The viscosity of the solutions was correlated with the solution density for different compositions. It was found that solutions of the same density have different viscosities, depending upon the carbon dioxide concentration in the mixture. The solutions with the higher carbon dioxide content display the lower viscosities at a given density. (C) 2000 John Wiley & Sons, Inc.