The mech. stability of biofilms and other microbial aggregates is of great importance for both the maintenance of biofilm processes and the removal of undesired biofilms. The potential binding forces are weak interactions such as London dispersion forces, electrostatic interactions and hydrogen bonds. In a first attempt to elucidate their contribution, extracellular polymeric substances (EPS) from a mucoid strain of Pseudomonas aeruginosa are studied by solid state NMR and rheol. measurements. Polyacrylic acid is identified as a suitable model system for EPS when the influence of electrolytes on mol. interactions is obsd. Effects on biofilm solns. and model systems are monitored by solid state NMR techniques, viscosimetry and dynamic light scattering. Based on the results, electrostatic interactions and hydrogen bonds are identified as the dominating forces among macromols. within the biofilm.