Miscibility and phase behavior of solutions of polyethylene (PE) and poly(dimethylsiloxane) (PDMS) mixtures in near-critical n-pentane have been investigated in a special variable-volume view-cell equipped with a computerized data acquisition system. This is a study on dissolving mutually incompatible polymers in a common solvent at high pressures. The fluid-fluid and fluid-solid demixing pressures of the solutions were determined for different polymer concentrations (5% PE, 5% PE+1% PDMS, 5% PE+2% PDMS and 5% PE+5% PDMS). In the PE+n-pentane solutions, the system shows LCST (lower critical solution temperature) type behavior and the fluid-fluid demixing pressures increase with increasing temperature. The PE+PDMS+n-pentane systems, however, show UCST (upper critical solution temperature) type behavior and the fluid-fluid demixing pressures decrease with increasing temperature. Even with small addition of PDMS to PE, the demixing pressures show dramatic increases compared to the demixing pressures of PE alone. At high PDMS concentrations (5% PDMS), complete miscibility could not be achieved at pressures up to 70 MPa. The fluid-solid boundary that is associated with the melting or crystallization of PE was also studied as a function of cooling and heating rates. It is shown that these temperatures tend to approach the same value in the limit of very low heating and cooling rates.