The demixing processes that occur during the polymerization of styrene in the presence of a low molar mass polyethylene wax were investigated. Quantitative information on the phase behavior of such a three-component system was obtained through the investigation of the temperature-induced phase separation and the observation of the phase separation during polymerization. Both techniques allow the construction of the same ternary phase diagram. Such phase behavior can be understood through the discussion of the interference of a liquid-liquid phase-separation process and crystallization. (C) 2003 Wiley Periodicals, Inc.