The densities of two polymer/CO2 single-phase solutions, poly(ethylene glycol) (PEG)/CO2 and polyethylene (PE)/CO2, were measured at temperatures higher than melting temperature of the polymer under CO2 pressures in the range 0-15 MPa using a newly-proposed gravimetric method. A magnetic suspension balance (MSB) was used for the density measurement under the high pressure CO2: A thin disc-shaped platinum plate was submerged in the considered polymer/CO2 single-phase solution in the MSB high-pressure cell. The weight of the plate was measured while keeping CO2 pressure and temperature in the sorption cell at a specified level. Since the buoyancy force exerted on the plate by the polymer/CO2 solution reduced the apparent weight of the plate, the density of the polymer/CO2 solution could be calculated by subtracting the true weight of the plate from its measured weight. Experimental results showed that the density of PE/CO2 solution increased with the increase of CO2 pressure and the density of PEG/CO2 solution decreased with the increase of CO2 pressure. To differentiate the effect of CO2 dissolution in polymer from that of mechanical pressure, the density of polymer/CO2 solution was compared with the density of neat polymer under the given mechanical pressure, which was calculated using the Sanchez-Lacombe equation of state and Pressure-Volume-Temperalure data of the polymer. The comparison could elucidate that the dissolution of CO2 in polymer reduced density of both PEG/CO2 and PE/CO2 systems but the degree of CO2 induced-density reduction was different between two polymer/CO2 systems. (C) 2007 Wiley Periodicals, Inc.