In this paper a new method based on the determination of heat capacity increment at the glass transition (Delta Cp) is presented to quantify the effectiveness of compatibilizers for immiscible polymer blends. In order to show the validity of the method, two immiscible blends, polypropylene-poly(ethylene terephthalate) (PP-PET) and PP-polyamide-6,6 (PP-PA66), and two compatibilizers, N, N-dihydroxyethyl monomaleic amide-grafted PP (g-PP) alone and together with a phenolic resin (PR), were investigated. Scanning electron microscopy (SEM) observations prove that the two compatibilizer systems are both effective for compatibilizing the blends, and the combined use of g-PP and PR is more effective. than g-PP alone. Modulated-temperature differential-scanning calorimetry (M-TDSC) determinations reveal that the Delta Cp varies with the extent of compatibilization. For the uncompatibilized blends, the Delta Cp for the PET component in PP-PET or for the PA66 component in PP-PA66 was found to be almost unchanged. After compatibilization these quantities become smaller. Also, the combined use of g-PP and PR results in the smallest Delta Cp values for both blends. This Delta Cp change with different compatibilizers is in very good agreement with the corresponding morphological variation observed by SEM. Thus, Delta Cp can be taken as a new parameter for quantifying the extent of compatibilization, since it is a direct measure of interfacial content.