The interaction of formate and acetate ions with cobalt-substituted carbonic anhydrase (CA) has been investigated through 13C-NMR and one-dimensional and two-dimensional 1H-NMR spectroscopy. 13C data on formate are consistent with a regularly coordinated ligand, as previously proposed for the acetate anion [Bertini, I., Luchinat, C. & Scozzafava, A. (1977) J. Chem. Soc. Dalton Trans., 1962-1965]. 1H-NOE experiments on both anions give evidence of through-space interactions between ligand protons and protein protons. The latter are assigned to specific residues in the active cavity through nuclear Overhauser effect spectroscopy (NOESY) experiments. The 13C-derived and 1H-derived constrains allow reliable docking of these ligands in the active-site cavity. The resulting geometries are similar to one another and consistent with five-coordinated structures around the metal ion, as previously proposed from electronic spectroscopy [Bertini, I., Canti, G., Luchinat, C. & Scozzafava, A. (1978) J. Am. Chem. Soc. 100, 4873-4877]. The results are discussed in light of the current debate on anion binding to metal ions in carbonic anhydrase [Lindahl, M., Svensson, A. & Liljas, A. (1992) Proteins, in the press]; Bertini, I., Luchinat, C., Pierattelli, R. & Vila, A. J. (1992) Inorg. Chem., in the press; Banci, L. & Merz, K. (1992) unpublished results] and, in particular, of the proposed long Zn-O distance found in the recent X-ray results on the formate adduct [Hakanson, K., Carlsson, M., Svensson, A. & Liljas, A. (1992) J. Mol. Biol., in the press].