The diffusion properties of caffeine and dextran within chitosan networks with controlled structure were investigated using a gel-gel method and fluorescence recovery after photobleaching (FRAP) by fringe pattern. The center of mass diffusion was studied by varying the molecular weight of dextran macromolecules labeled with fluorescein isothiocyanate (FITC). It was shown that the diffusion was drastically slowed down above a critical molecular weight around 10(5) g/mol.