Blend phase characterization is a fundamental step in a polymer blend study. To analyze the blend composition and morphology, the separation of thermoplastic polymer blends into its components is the method used. This separation by traditional techniques, such as Soxhlet, is very difficult because of the high molecular weight molecules of the polymer blend components. The use of a high-temperature-high-pressure near-critical solvent is an interesting alternative that results in a complete and fast separation. In this work, a systematic study of the demixing of polypropylene (PP)/polystyrene ( PS) blends is presented. First, the selectivity of n-alkanes was explored at high pressure and over a wide range of temperatures on pure polymers. From this study the processing window for blend separation was obtained. Second, the influence of the blend morphology and composition on the separation efficiency was analyzed. The method was applied to analyze and quantify the copolymer formed in a PP/PS in situ compatibilized blend. It is concluded that the method proposed for blend separation is simple, fast, and accurate for the systems analyzed.