We have studied metal ion complexation with poly(carboxylic acid) ligands using density functional methods and a continuum solvation model (COSMO). Geometry optimisations have been carried out for metal complexes of poly(acrylic acid) (PAA) and poly(α-hydroxy acrylic acid) (PHA) oligomers. The complexation energies for Mg2+, Ca2+, Mn 2+, Fe3+, and Zn2+ with one oligomer ligand (pentamer), and two ligands (dimer and trimer) have been calculated. The nature of hydrogen bonding in PHA has been studied both in free oligomers and in metal complexes. The obtained binding degrees are in good agreement with experimental results although with quite small energy differences. The complexation energies were found to be very sensitive to changes in internal hydrogen bonds. The role of hydroxyl oxygen atoms in metal ion binding was observed to be more important for Ca2+ than for other metals.