High-pressure viscosity and density of solutions of poly(methyl methacrylate) (PMMA) in acetone and in acetone + CO2 mixtures have been determined in the temperature range from 50 to 125 degrees C in steps of 25 degrees C over the pressure range from 7 to 35 MPa in steps of 7 MPa using a failing cylinder type viscometer. Measurements were conducted with PMMA samples of two different molecular weights (M-w = 15,000, M-w/M-n = 1.8 and M-w = 540,000, M-w/M-n = 2.8) at concentrations of 2, 5, 10 and 20 wt.% with M-w=540,000, and at concentrations of 10 and 20 wt.% with M-w = 15,000 sample. The effect of adding CO2 on viscosity was investigated for the 5 wt.% solutions with the high molecular weight polymer sample. The viscosities were observed to be relatively low, with values in the range from 0.2 to 1.6 mPa s for these solutions. The flow activation energies were around 5-10 kJ/mol. Flow activation volumes were in the range from 5 to 40 cm(3)/mol. Close-packed volumes determined from density correlations were in the range of 0.76-0.98 cm(3)/g.. The overlap concentrations, c*, were estimated to be in the range 0.045-0.075 g/cm(3). In the presence of CO2, densities of the solution show an increase, reflecting the higher density of compressed CO2 compared to that of acetone, but viscosities were significantly lowered, with a reduction of about 30% at 4 wt.% CO2 additions. The density dependence of viscosity is used to visually illustrate the need for higher pressures in the presence Of CO2 to sustain a given viscosity level. (C) 2006 Elsevier B.V. All rights reserved.