The coil-to-globule and globule-to-coil transition of poly(N-isopropylacrylamide) in aqueous solution had been studied by heating and cooling the sample solution with conventional viscosity measurement. A single chain collapsed globule solution was prepared firstly by adding sodium n-dodecyl sulfate (SDS) into the polymer solution at room temperature, as the chain collapsed to compact globule at higher temperature and then the SDS was removed by electro-dialysis. The viscosity data were analyzed in a quantitative way, which permitted to elucidate the transition temperature and the amount of the water in the collapsed globule precisely. (C) 2003 Elsevier Ltd. All rights reserved.