The ultrasonic properties of poly(N-isopropyl acrylamide) (PNIPAM)/water solutions, determined with high-resolution ultrasonic spectroscopy (HR-US), change during demixing and remixing. All HR-US measurements are discussed with respect to modulated temperature differential scanning calorimetry results. The lower critical solution temperature type of phase behavior, in combination with the glass-transition/composition curve of PNIPAM/water, determines the evolution of the ultrasonic signals. Three different temperature regions can be distinguished: a homogeneous region and a heterogeneous region, the latter subdivided into zones without and with interference of partial vitrification of the PNIPAM-rich phase. During phase separation, the ultrasonic velocity decreases because of a change in the hydration structure around the polymer chains, whereas the ultrasonic attenuation increases as aggregation sets in. Isothermal measurements clearly show time dependence for both the velocity and the attenuation. The observed timescales are different and can be related to a changing polymer/water interphase and aggregate formation, respectively. Partial vitrification of the PNIPAM-rich phase slows the demixing kinetics and especially the remixing kinetics. © 2005 Wiley Periodicals, Inc.