The coordination of bicarbonate to Mn2+ is the simplest model system for the coordination of Mn2+ to carboxylate residues in a protein. Recently, the structure of such a complex has been investigated by means of X-band pulse EPR (electron paramagnetic resonance) experiments (Dasgupta, J.; et al. J. Phys. Chem. B 2006, 110, 5099). Based on the EPR results, together with electrochemical titrations, it has been concluded that the Mn2+ bicarbonate complex consists of two bicarbonate ligands, one of which is monodentate and other bidentate, but only the latter has been observed by the pulsed EPR techniques. The X-band measurements, however, suffer several drawbacks. (i) The zero-field splitting (ZFS) term of the spin Hamiltonian affects the nuclear frequencies. (ii) There are significant contributions from ENDOR (electron nuclear double resonance) lines of the M-S not equal +/- 1/2 manifolds. (iii) There are overlapping signals of Na-23. All these reduce the uniqueness of the data interpretation. Here we present a high-field ENDOR investigation of Mn2+/(NaHCO3)-C-13 in a water/methanol solution that eliminates the above difficulties. Both Davies and Mims ENDOR measurements were carried out. The spectra show that a couple of slightly inequivalent C-13 nuclei are present, with isotropic and anisotropic hyperfine couplings of A(iso1) = 1.2 MHz, T-perpendicular to 1 = 0.7 MHz, A(iso2) = 1.0 MHz, T-perpendicular to 2 = 0.6 MHz, respectively. The sign of the hyperfine coupling was determined by variable mixing time (VMT) ENDOR measurements. These rather close hyperfine parameters suggest that there are either two distinct, slightly different, carbonate ligands or that there is some distribution in conformation in only one ligand. The distances extracted from T-perpendicular to 1 and T-perpendicular to 2 are consistent with a monodentate binding mode. The monodentate binding mode and the presence of two ligands were further supported by DFT calculations and H-1 ENDOR measurements, Additionally, Na-23 ENDOR resolved at least two types of Na-23(+) in the Mn2+-bicarbonate complex, thus suggesting that the bicarbonate bridges two positively charged metal ions.