Pulsed field gradient spin echo (PFG-SE) NMR technique has been used to measure the self-diffusion coefficient (D-self) of water in a thermoreversible hydrogel, Poly(N-isopropylacrylamide). D-self, was measured as a function of temperature (-5-50 degrees C) and hydration level. A theoretical model to predict D-self was built by using a combination of lattice fluid hydrogen bond theory and free volume theory. The fully swollen gel did not show any discontinuous change in D-self with temperature, Whereas, a partially swollen gel, showing the existence of two kinds of water having diffusivities differing by order of magnitude, exhibits a discontinuous change in D-self. These results were explained within the theoretical framework developed by us. The present work thus enables an elucidation of diffusion phenomenon at the volume transition point for the first time.