AbstractThe stability constants of the methylmercury complexes CH3HgL1−λwith a considerable number of ligands Lλ−have been determined (Table 1). Like H+, the cation CH3Hg+reveals the coordination number 1 almost exclusively. Some ligands form also binuclear adducts (CH3Hg)2L2−λand sulfide even forms (CH3Hg)3S+. The coordination behaviours of CH3Hg+and H+however are very different concerning the selection of ligands, the former being a soft and the latter a hard acid in the sense of PEARSON. The entropy ΔS of the reaction: H++ L−→ HL is more positive by about 14 c.u. than ΔSof the reaction: CH3HG++ L−→ CH3HgL. The hydrogen ion in aqueous solution has a far greater order producing action on the solvent than CH3Hg+. Two dyes also have investigated, which may serve as indicators for CH3HG+and for the determination of pCH3H