A potentiometric and H-1 NMR investigation on the interactions between Hg2+ and some biologically important sulfhydryl ligands, such as cysteine (H2CYS), penicillamine (H2PSH), and glutathione (H(3)GSH), is reported. Equilibria were studied at T = 298.15 K, in the ionic strength range (0.1 to 1) mol kg(-1), using as ionic medium NaCl in the presence of iodide (Nal) as a competitive ligand. Results show the formation of HgL2-z, HgLH3-z, HgLH24-z, HgL22-2z, HgL2H(3-2z), and HgL2H24-2z species (Lz- = CYS2-, PSH2-, or GSH(3-)) together with HgL2H3- for GSH(3)(-) only. Formation constant values are very high with, as an example, log beta = 34.54, 34.24, and 32.05 for Hg(CYS)(0), Hg(PSH)(0), and Hg(GSH)(-) species, respectively (at I = 0.25 mol kg T = 298.15 K). H-1 NMR measurements at I = 0.25 mol.kg(-1) (NaCl) fully confirm the potentiometric findings. The speciation diagrams show that most of the metal fraction is present as complex species in a wide pH range, and the hydrolysis of the cation is completely suppressed. The sequestering ability of ligands toward Hg2+ is very high, and it was analyzed and compared at different pH and ionic strengths.