New mercury organic compounds with 1-3 metal centers and different substituents were synthesized as anion-selective ionophores. They were incorporated into solvent polymeric membranes, and the selectivity, slope of response, response time, emf repeatability, and the signal stability of the corresponding ion-selective electrodes (ISEs) were studied. Ortho-dimercury aromatic compounds were found to induce an adequate chloride-selectivity for physiological measurements. The anion complexation behavior of the ionophores in organic phases was investigated by 13C-NMR-monitored titrations with different tridodecylmethylammonium salts. Generally, the mercury organic compounds were shown to act as electrically neutral ionophores, forming negatively charged complexes with anions (one anion bound per mercury atom). A preference of chloride and thiocyanate over nitrate and perchlorate was documented. In two-phase experiments with 1R and 199Hg-NMR detection, the chemical stability of the ionophores during exposure to different aqueous electrolytes was investigated. Compounds with native acetoxy or trifluoroacetoxy substituents at the mercury centers showed a limited stability in contact with chloride solutions. Structural changes of the ionophores due to an exchange of substituents were observed, which were responsible for potential instabilities of the respective ISEs. The mercury organic compound ETH 9033 with chloro-substituents was most stable, and ISEs based on it showed the best performance.