Isothermal titration calorimetry (ITC) coupled with structural characterization methods was used to characterize a heterogeneous mixture of polyphenolic compounds obtained from the chemical modification of lignin from a residual waste product. The mixture could be represented effectively as a single pseudo-compound (C20H19O14N2) with well-defined molecular weight (525 g/mol). It has three acidic sites with pK(a) values at 2.7, 4.8 and 10.8. The complexation with all assayed cations yielded a order Cu > Pb > Mg and metal ion complexation stoichiometry of two metal ions per lignin molecule. Formation enthalpies and entropies were also determined. (C) 2003 Elsevier Science B.V. All rights reserved.