High resolution and high sensitivity ultrasound resonator technology offers unique possibilities for the investigation of polymer precipitation. Exemplarily, the aggregation of poly(N-isopropylacrylamide) units in block copolymers with poly(styrene sulfonate) is investigated. Long term changes, over hours and days, of both the ultrasound velocity and the attenuation were detected during isothermal studies at temperatures in the vicinity (below as well as above) of the critical solution temperature. Apparent activation free energies of the precipitation and re-dissolution of the poly(N-isopropylacrylamide) block have been determined for the first time. The apparent activation energies are in the order of up to a few thousands kJ/mol, which can be explained by a high cooperativity of the precipitation process. (C) 2008 Elsevier Ltd. All rights reserved.