The pH dependent complexation between copper and C-13-labeled carbonate ligands in aqueous solution is investigated by optical spectroscopy and continuous wave and pulse EPR. The small C-13 hyperfine coupling observed in the pulse EPR spectra at low temperature and pH 5.5 is assigned to weak, monodentate coordination of carbonates to the Cu2+ ion. The C-13 hyperfine couplings found at pH 6.5 and 8 are of the same magnitude as twice the nuclear Larmor frequency (matching range) and are assigned to bidentate coordination of carbonate. The larger coupling for the bidentate complexes is explained by a higher covalency of the it bonding in the four-membered ring. The elongation of the axial bonds to the water Ligands caused by coordination of carbonate ligands in the equatorial plane of the copper complex leads to smaller proton couplings. The observation of both mono-and bidentate coordination at pH 6.5 and 8 by pulse EPR at low temperature is in contradiction to the calculated concentrations of copper-carbonate complexes at room temperature. Although equilibrium shifts during the freezing of the samples cannot be excluded, the basic mechanisms of mono-and bidentate complexation observed in frozen solution should also apply in aqueous solution at ambient temperature.