The viscosity and speed of sound were measured in a poly(N-isopropylacrylamide)-water system that exhibited a lower critical solution temperature. The measurements were performed in a polymer concentration range of 0.9-10 wt % and at temperatures ranging from 295 K to the demixing temperatures. The temperature dependence of the viscosity showed an unusual behavior in which three parts could be distinguished. Far from the demixing temperatures, the viscosity decreased with the temperature according to an Arrhenius relation. As the demixing points were approached, an intermediate region, weakly dependent on the temperature, was observed, and this was followed by a strong increase in the viscosity ending abruptly at a higher temperature. These breakdown points were identified with the demixing points and agreed very well with the cloud points determined by the optical method. The behavior of the viscosity close to the demixing points was believed to be determined by the strong critical effects. The concentration dependence showed an anomaly centered around 3%. Furthermore, the relative change of the viscosity with the temperature as a function of the concentration also showed an anomaly close to a concentration of 3%. The description of the critical behavior of the viscosity in terms of power laws gave a critical viscosity exponent equal to 0.056, which was much higher than what was theoretically expected. The use of a more general equation suggested a weak power divergence. The temperature dependencies of the speed of sound exhibited abrupt changes at temperatures corresponding to the phase transition. The agreement with optical cloud points and viscometric demixing points was very good. An exceptionally large isotope effect on the viscosity was observed, whereas that for the speed of sound was markedly lower and was in agreement with the expected value. These findings were qualitatively examined. (C) 2003 Wiley Periodicals, Inc.