Structure and molecular mobility of complexes between carboxymethylpolysaccharides (carboxymethylcellulose and carboxymethylchitin) and alkaline‐earth metal ions in aqueous systems were studied by means of dynamic viscoelasticity, SAXS and NMR measurements. The frequency‐dependent curves of the dynamic modulus measured with a cone‐plate rheometer shift to a longer time‐scale region with complex formation and can be superposed to a master curve by only a horizontal shift. This means that the relaxation times lengthen by complexation and that the complexation is an intramolecular rather than an intermolecular process. The SAXS and NMR measurements revealed that the site for complexation is a cavity between the carboxyl group and the hydroxy group attached to the neighboring residue. A new molecular model for complexation was presented.