The phase behavior and volumetric properties of polyethylene (PE) in solutions of n-pentane and n-pentane/CO2 were studied in a temperature (T) range of 370-440 K at pressures up to 60 MPa. Measurements were conducted with a variable-volume view-cell system equipped with optical sensors to monitor the changes in the transmitted light intensity as the P or the T of the system was changed. Lower-critical-solution-temperature-type behavior was observed for all of the liquid-liquid (L-L) phase boundaries, which shifted to higher pressures in solutions containing CO2. The solid-fluid (S-F) phase boundaries were investigated over a P range of 8-54 MPa and took place in a narrow T range, from 374 to 378 K in this P interval. The S-F phase boundary showed a unique feature in that the demixing temperatures showed both increasing and decreasing trends with P depending on the P range. This was observed in both the PE/n-pentane and PE/n-pentane/CO2 mixtures. The density of these solutions were measured as a function of P at selected temperatures or as a function of T at selected pressures that corresponded to the paths followed in approaching the phase boundaries (S-F or L-L) starting from a homogeneous one-phase condition. The data showed a smooth variation of the overall mixture density along these paths. (C) 2003 Wiley Periodicals, Inc.