Arguably the most important property of a hydrogel is the ability to allow the diffusion of solutes through the crosslinked network. Studies of the diffusion in hydrogels are important for providing information on the rate and extent of the passage of the solute and on the details of the microstructure of the hydrogel. Such knowledge is directly relevant for applications such as controlled drug delivery systems. The structure of novel poly(N-isopropylacrylamide) (PNIPAAm) hydrogels can be revealed by the restricted diffusion of appropriate probe molecules. Dextran molecules, labelled with fluorescent moieties, were incorporated into well-defined PNIPAAm hydrogels to investigate the effects of hydrogel mesh size and dextran molecular size on the diffusivities of solute molecules.