The present contribution aims at testing experimentally the theoretical model previously devised (Donati, I.; Cesaro, A.; Paoletti, S.; Biomacromolecules 2006, 7, 281-287) for the description of the interaction between alginate and nongelling Mg2+ ions. The model, based on an extension of the counterion condensation theory, introduces a contribution of free energy of affinity, (Delta G) over bar (aff,0), which depends on the monomer composition of the polyuronate. In the present work, three different alginates separately mimicking the mannuronan (polyM), the guluronan (polyG), and the polyalternating (polyMG) components of alginate, together with a natural alginate isolated from Laminaria hyperborea (L. hyperborea), were examined. They were treated with Mg2+ ions, and relative variations in scattered light intensity, isothermal calorimetry (Delta H-mix) specific viscosity, and Na-23 NMR longitudinal relaxation rates were monitored with respect to samples at the same ionic strength but containing only Na+ ions. The fraction of condensed magnesium counterions was found to be strongly dependent on alginate composition, increasing along the series mannuronan < polyalternating similar to L. hyperborea < guluronan, thus in good agreement with the theoretical predictions.