Solubility of polyethylene molecular weight standards (M(w) = 2150, 16,400, 108,000, and 420,000 and M(w)/M(n) = 1.14, 1.16, 1.32, and 2.66, respectively) has been studied in near- and supercritical n-butane and n-butane/CO2 mixtures at pressures up to 70 MPa. For each polyethylene / solvent system at selected compositions, demixing pressures have been determined using a high-pressure variable-volume view-cell at temperatures up to 200-degrees-C. Solutions in pure n -butane are found to display LCST (lower critical solution temperature)-type behavior. The behavior of the solutions in n-butane/CO2 mixtures are observed to change from the LCST to the UCST (upper critical solution temperature) with increasing CO2 content in the binary solvent. Sanchez-Lacombe theory has been used to model these systems. The predictions correctly describe the nature of the phase diagrams for both binary and ternary systems and the calculations are in reasonable agreement with experimental data.