The ionization constants of four recently prepared thioethercarboxylic acid chelating agents have been determined along with the stability constants of the complexes of these chelating agents with Hg2+, Zn2+, Ca2+, Ni2+, Mg2+, Cd2+, and Cu2+. The marked selectivity of these chelating agents for Hg2+ is quite striking. For ions with which a bond to a thioether sulfur is not strong, the stability constants are fairly low. A combination of pH measurements and pM measurements were used to determine the stability constants of the species MY and MHY. Because of the geometry of these chelating agents the proton in MHY is far removed from the metal bonding site and a consequence of this is the fact that the stability constants for MY and MHY may be quite close in some cases.