A new chelating agent, phthalytetrathioacetic acid, (PTTA), has been characterized and found to be highly selective for Hg2+. All four of its ionization constants have been determined and the stability constants of its complexes with Hg2+, Ca2+, Zn2+, Co2+, Pb2+, Ni2+, and Cd2+ estimated. The mercury complex has a stability constant greater than those of the other complexes by a factor of at least 1010. The high selectivity for the Hg2+ ion is due to a closely spaced group of thioether sulfurs and a rather wide separation of the carboxylic acid groups which are potential members of chelate rings. The mercury complexes present in aqueous solution are Hg(PTTA)H2, log K = 18·8 at low pH and Hg(PTTA) (OH)2, log K = 45 at high pH. The presence of thioether, rather than thiol sulfur atoms has the result that the chelate with Pb2+ is of low stability.