A study on chemical equilibria of different Hg(2+)-organic ligand systems is reported to establish the sequestration power of N/O donor groups versus metal. Ligand classes considered are: O-donors (succinate, 1,2,3,4-butanetetracarboxylate, 1,2,3,4,5,6-benzenehexacarboxylate, polymethacrylate, and polyacrylates); N-donors (ethylenediamine, diethylenetriamine, spermidine, triethylentetramine, tetraethylenpentamine, poly(allylamine), and poly (ethylenimine)); and amino acids (glycine, histidine, and aspartic acid). Equilibria were studied potentiometrically in NaNO(3) at I = 0.1 mol.L(-1) and t = 25 degrees C. For some systems, the dependence on ionic strength in the range (0 <= I <= 1) mol.L(-1) (NaNO(3)) and the overall enthalpy changes, by calorimetric measurements, were also determined. The sequestering ability of ligands toward Hg(2+) was evaluated, defining the amount of ligand necessary to sequester 50 % of the total metal cation concentration, pL(50). The dependence on temperature and on ionic strength of pL(50) was determined. General trends for the formation parameters for Hg(2+)-O-donor and -N-donor species depending on the binding sites were found. The free energy of binding for both unprotonated and monoprotonated species of carboxylates (Delta G(b) = -21.7 +/- 3 kJ.mol(-1)) and for unprotonated species of amines (Delta G(b) = -35.4 +/- 5 kJ.mol(-1)) was estimated.