Carboxylate (COO − ) groups can coordinate to metal ions in of the following four modes: ‘unidentate’, ‘bidentate’, ‘bridging’ and ‘pseudo-bridging’ modes. COO − stretching frequencies provide information about the coordination modes of COO − groups to metal ions. We review the Fourier-transform infrared spectroscopy (FTIR) of side-chain COO − groups of Ca 2+ -binding proteins: pike parvalbumin pI 4.10, bovine calmodulin and Akazara scallop troponin C. FTIR spectroscopy of Akazara scallop troponin C has demonstrated that the coordination structure of Mg 2+ is distinctly different from that of Ca 2+ in the Ca 2+ -binding site. The assignments of the COO − antisymmetric stretch have been ensured on the basis of the spectra of calcium-binding peptide analogues. The downshift of the COO − antisymmetric stretching mode from 1565 cm -1 to 1555–1540 cm −1 upon Ca 2+ binding is a commonly observed feature of FTIR spectra for EF-hand proteins.