The recent introduction of modulated-temperature differential scanning calorimetry (M-TDSC) has greatly extended the power of calorimetry as a characterization technique for polymers. A method which can be used to study macromolecular diffusion in the interfacial phase of core-shell latex particles after film formation has been derived. Here, the extent of macromolecular diffusion with time of annealing at 150 degrees C in a poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc) core-shell latex has been studied by means of modulated-temperature differential scanning calorimetry (M-TDSC). By measuring the change of increment of heat capacity in the glass transition region, the total interface content can be determined. For film integration of core-shell latices, M-TDSC gives quantitative information on the formation of a diffusion interface. According to the M-TDSC measurements of the weight fraction, interfacial thickness and its development with time can be determined. With increasing time, the interfacial thickness between the core and shell phases decreased. The PMMA diffusion coefficient is approximately 4 x 10(-14)cm(2)/s. Reptation behaviour of macromolecular diffusion in phase separation was confirmed experimentally.