Biofilms, the predominant life-form of most microorganisms, can bind large amounts of water by forming a matrix of Extracellular Polymeric Substances (EPS). This water binding ability is mainly attributed to the formation of polysaccharide hydrogels. In this study the influence of the extracellular polysaccharide alginate on the water mobility was investigated by different NMR methods. By PFG-NMR it was shown, that aqueous solutions of algal alginate contain a fraction of water with a significantly lower diffusion coefficient compared to the bulk water. This behaviour could be led back to the hindered diffusion of water encapsulated in alginate/protein aggregates. A mathematical model allowed the determination of the aggregate sizes from the NMR data, which were in good agreement with the results of laser diffraction experiments. On bacterial alginate of Pseudomonas aeruginosa strong cross relaxation effects between water and acetyl protons were observed by stimulated echo and 2D-NOESY experiments. A pool of hydration water associated with the acetyl groups by hydrogen bonds could be assumed by these results. Dried samples of algal alginate showed a strong impact of alginate/protein clusters on the water mobility. These clusters provided entrapped water with high mobility. Aggregation of alginate with lectines could be proved. These aggregates encapsulate water, which shows the same diffusional behaviour as the water in the above mentioned natural clusters. This aggregation is based on ionic interactions between the alginate´s carboxylate groups and the lectine´s positive amino acid residues. The results of this thesis led to the conclusion, that the binding between saccharides and proteins can have an important impact on the water retention ability of biological systems.