The effect of bivalent cations on solns. of extracellular polymeric substances (EPS) isolated from Pseudomonas aeruginosa was monitored by means of solid-state NMR. In particular, the binding of Ca2+ and Mg2+ to the alginate in aq. soln. was studied by detg. the spin-lattice relaxation rates, line widths and line shapes of 13C nuclei under variation of the ion concn. 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.