It was reported previously that N-benzylethylenediamine triacetic acid (Ⅰ), a pentadentate chelating agent, formed a rather unstable antimony complex^[2].Since thioether is regarded as a donor group, and N-(β-methylthioethyl)N,N-diacetic acid (Ⅳ) formed more stable metal chelates than that of N-methyl-N,N-diacetic acid (Ⅴ)^[8], it appears that the introduction of a thioether group into the ethylenediamine triacetic acid molecule would increase the stability of the antimony chelate by virtue of the formation of an additional coordination bond between sulphur and antimony atoms.For the same reason β-mercaptoethylamine triacetic acid (Ⅶ), the compound resulted from the replacement of one nitrogen atom by sulphur in the ethylenediamine triacetic acid molecule, would be a pentadentate chelating agent for antimony.