The viscosity of solutions of polystyrene (PS) in decahydronaphthalene (DHN) was measured in the presence of carbon dioxide (CO,) with a moving-piston viscometer. The effects of the CO, pressure (0-21 MPa), polymer concentration (1-15 wt %), temperature (306-423 K), and polymer molecular weight (126 and 412 kDa) on the viscosity were investigated. In the absence of CO2, the increase in the viscosity with increasing polymer concentration was approximately exponential in concentration and was well described by the Martin equation. All the data fell oil a sin le line when the relative viscosity was plotted against cM(0.50) (where c is the concentration of the polymer ill solution and M is the molecular weight of the polymer). The viscosity of the polymer solution decreased with increasing CO2 pressure under otherwise constant conditions. For a given CO2 pressure, the viscosity reduction was greatest when the relative viscosity was high in the absence of CO2 that is, for high-molecular-weight polymer, high polymer concentrations, and low temperatures. Reductions in the relative viscosity of almost 2 orders of magnitude,were observed in some cases. The viscosity of solutions of PS in DHN also was measured in the presence of sulfur hexafluoride (SF6). At a given pressure, SF6 was about as effective as CO2 in reducing the solution viscosity. (c) 2005 Wiley Periodicals, Inc.