The two species, Fe(OH2)Y- and Fe(OH)Y2- of the iron(III) EDTA system in aqueous solutions were studied through their paramagnetic effect on the relaxation rates of 1H, 2H and 17O. The exchange mechanism of the bound water protons in Fe(OH2)Y- was found to involve the exchange of the whole water molecule with an exchange lifetime of (1.3 ± 0.4) × 10-6 s at 20°C. The proton exchange rate in the basic form Fe(OH)Y2- is much slower, with a residue time of (8 ± 3) × 10-6 s, at 20°C. The effective distance of the dipolar interaction between the unpaired electrons and the bound water protons in Fe(OH2)Y- and the hydroxylic proton in Fe(OH2)Y2- was shorter than that expected on the basis of crystallographic data, but it was almost the same for the two species. The relevance of these results to the NMR evaluation of binding of hydroxide ion to the metal in the enzyme carbonic anhydrase is discussed.