Formation of ternary complexes between Gd-DTPA, Gd-DTPA-BMA, and Gd-DOTA, used as contrast enhancement agents in MRI and the endogenously available carbonate and phosphate ions, has been demonstrated. The extent of ternary complex formation and its effect on the proton relaxation, measured at 9 MHz, rates is negligible at around pH < 8. The complex Gd-EDTA forms more stable ternary complexes with carbonate and phosphate and it also strongly coordinates the terdentate citrate ligand. The formation of ternary complexes Gd-EDTA(X) (X = CO3(2-), Cit3-) results in a significant decrease in the proton relaxation rates under physiological conditions.