Phase separation of two partially miscible polymer blends is studied with modulated temperature DSC (MTDSC). The lower critical solution temperature (LCST) demixing behavior of poly(ethylene oxide) (PEO) blended with poly(ether sulphone) (PES) and with poly(3,4'-diphenylene ether isophtaloyl amide), as determined by cloud point temperatures with optical microscopy, is in excellent agreement with results obtained from non-isothermal MTDSC measurements. The non-isothermal MTDSC apparent heat capacity evolution is time-dependent. It is influenced by the endothermic demixing enthalpy and, in the case of PEO/PES blends, by the vitrification of a high-T-g phase formed. Quasi-isothermal MTDSC measurements contain information on the kinetics of demixing and remixing, emphasizing the added value of MTDSC to follow in situ the diffusion-controlled phase separation processes of partially miscible polymer blends.