Demixing conditions were measured visually for solutions of three narrowly distributed polyethylene samples (M ranging from 6.5 to 380 kg/mol) in n-hexane up to 500 K and 150 bar. This information yields the critical line for infinite molar mass; i.e., it specifies the pT area within which one is safe from phase separation irrespective of the molar mass of the polymer and of the composition of the mixture. The experimental findings are in good qualitative agreement with the predictions of the Sanchez-Lacombe theory applied without any adjustable parameters. The influences of molar masses are, however, underestimated, and the resulting critical compositions are too small. For a more quantitative description, the observed phase behavior of the system was modeled by means of the Flory-Huggins theory, employing a new expression for the concentration dependence of the interaction parameter, which is based on chain connectivity and conformational relaxation. With this procedure, the required system-specific parameters are obtained from the critical conditions measured for different M values of the polymer. Despite the fact that only the data for the two higher molecular weight samples could be used for that purpose, the calculated binodals and spinodals appear reasonable.