The effects of nanoparticles and high-pressure carbon dioxide (CO2) on shear viscosity of polystyrene (PS) were studied. Master curves of PS, PS + 5 wt % carbon nanofibers (CNFs), and PS + 5 wt % nanoclay (Southern Clay 20A) without CO2 were created based on parallel-plate measurements. The results showed that addition of nanoparticles increased the viscosity of the neat polymer. Steady-state shear viscosity of PS in the presence Of CO2 and nanoparticles was measured by a modified Couette rheometer. The effect of supercritical CO2 Oil these systems was characterized by shift factors. It was found that under the same temperature and CO2 pressure, CO2 reduced the viscosity less for both PS-20A and PS-CNFs than neat PS. Between the two types of nanoparticles, CNFs showed a larger viscosity reduction than 20A, indicating a higher CO2 affinity for CNFs than 20A. However, the advantage of CNFs over 20A for larger viscosity reduction decreased with higher temperature. A gravimetric method (magnetic suspension balance) was used to measure the excess adsorption of CO2 onto CNFs and nanoclay, thus, CO2 showed a higher affinity for CNFs. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1068-1076, 2010