A quantitative thermal method of determining the weight fraction of interface and the extent of phase separation in polymer materials is described. It is based on the differential of heat capacity signal from modulated-temperature differential scanning calorimetry. By measurement of the increment of heat capacity at the glass transition temperature, the total interface content can be determined. The method assumes that the interface and the rest of the system can be modelled as a series of discrete fractions each with its own glass transition temperature. Several examples, including block copolymers, block copolymers blended with homopolymer, and two-phase and four-phase systems are given to illustrate the range of the method. The calculated results were close to the experimental data for two-phase and four-phase systems.