The heat capacity signal from modulated temperature DSC can be used to measure the onset of phase separation in aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions, showing a type II LCST demixing behavior. Quasi-isothermal measurements through the phase transition show large excess contributions in the (apparent) heat capacity, caused by demixing and remixing heat effects on the time scale of the modulation. These excess contributions and their time-dependent evolution are useful to describe the kinetics of phase separation and to follow the related morphology development. Partial vitrification of the polymer-rich phase slows down the remixing kinetics.