Time-resolved fluorescence anisotropy measurements have been performed upon an acenaphthylenelabelled (0.5 tool%) sample of poly(N-isopropylacrylamide), PNIPAM, in dilute solutions in both methanol and water as solvents. In methanol, segmental relaxation of PNIPAM follows an Arrhenius dependence upon temperature over the range investigated (279-323K) characterized by an 'activation energy' of 13.4(+0.5) kJ tool -l . This is only slightly greater (by ca 2.4 kJ mol 1) than that of solvent flow and it is likely that specific dipolar interactions between the PNIPAM and the methanol determine the macromolecular dynamics in this solvent. In aqueous solution, the segmental mobility of PNIPAM exhibits a dramatic thermoreversible discontinuity at ca 32ยฐC. This change in conformational behaviour occurs at the polymer's lower critical solution temperature in aqueous media. This observation, supports the proposition (Winnik, F. M. Polymer 1990, 31, 2125) that the thermally-induced separation in this system occurs by a 'dual mode' mechanism wherein intermolecular aggregation is preceded by intramolecular conformational shrinkage of the polymer coils.