The pK(a) values of curcumin and diacetylcurcumin are, here doubtless, determined by means of spectroscopic and potentiometric measurements, and the enolic proton is the more acidic one. The interaction of Fe3+ with curcumin and diacetylcurcumin, in water/methanol 1:1 solution, leads to the fort-nation of the complex species [FeH2CU(OH)(2)] and [FeDCU(OH)(2)] (H2CU and DCU = curcumin or diacetylcurcumin monoanion, respectively) which prevails near pH 7. At more basic condition the prevailing species are [FeH2CU(OH)(3)](-) and [FeDCU(OH)(3)](-), which prevent metal hydroxide precipitation. H-1 NMR data state that the dissociated P-diketo moiety of the ligands is involved in metal chelation. The pK(a) value of the deprotonation reaction is strongly anticipated by the metal ion, as shown by UV spectral data. The stability constants, evaluated from potentiometric data, are near to that of desferrioxamine, which is, by now, the only iron-chelating agent for clinical use. (C) 2002 Elsevier Science B.V. All rights reserved.