We have studied the interaction of a low molecular weight alginic acid (ca. 7,000 daltons) with Ca2+, Mg2+, Eu3+ and La3+, using lanthanide luminescence spectroscopy (LLS) and 13C NMR. LLS results imply that nine-coordinate lanthanide ions lose six water molecules upon complexation with alginate, forming inner-sphere alginate complexes. In general, the 13C resonances in the NMR studies decreased in intensity as metal ion concentration increased, but the preference of a cation for repeating mannuronate and guluronate units (MM and GG blocks) varied greatly. The La3+ ion binds preferentially to the GG blocks, though some binding to the MM blocks is evident, while Eu3+ binds equally well to GG and MM blocks. Ca2+ shows a strong preference for the GG blocks only, while Mg2+ shows no evidence of binding to alginate at all. These preferences may be explained in terms of cation size, charge and coordination number. The diequatorial linkage pattern of sequential mannuronate residues leads to a flat ribbon-like structure with shallow cavities for the metal ions while the diaxial linkages of the guluronates allow a deeper, more size-specific cavity. Calcium has the optimum ionic radius for the GG cavities compared to the larger Eu3+ and La3+ ions. The smaller Mg2+ ion does not bind at all over the concentration range studied. This could be due to its large charge density to coordination number ratio that would make dehydration of this ion and subsequent complexation by alginate thermodynamically unfavorable.