This paper discusses the dynamics of water molecules in poly(NIPAM) microgel dispersions. H-1 NMR spectroscopy is used to determine the self-diffusion coefficient of the water molecules. The capability of the microgel particles to restrict the motion of water molecules depends on the degree of swelling, as determined by dynamic light scattering. The dependence of molecular diffusion on the network density is established. This is tested against Yasuda's free volume model and Ogston's obstruction model, which have been modified to account for the dependence of the network pore size on the extent of swelling.