The effect of bivalent cations on solutions of extracellular polymeric substances (EPS) isolated from Pseudomonas aeruginosa was monitored by means of solid-state nuclear magnetic resonance. In particular, the binding of Ca2+ and Mg2+ to the alginate in aqueous solution was studied by determining the spin-lattice relaxation rates, line widths and line shapes of 13C nuclei under variation of the ion concentration. Both cations differ strongly in their affinity towards bacterial alginate. Spectral data indicate that the strong binding capacity of calcium is connected to the formation of a chelate complex, in which binding occurs particularly with the monomer units in alternating mannuronate-guluronate blocks. In contrast to this, binding of magnesium ions was found to be much weaker and non-specific.