Statistics for A simple and selective complexometric method for the determination of thallium(III) is proposed by using thioglycolic acid as a masking agent. In the presence of diverse metal ions thallium(III) is complexed with excess EDTA and the surplus EDTA is back titrated (pH 5 - 6 , hexamine) with standard zinc sulphate solution using xylenol orange indicator. A freshly prepared 1% aqueous solution of thioglycolic acid is then added to displace EDTA from Tl(III)-EDTA complex and the released EDTA is titrated with standard zinc sulphate solution. The results for the determination of 4 - 87 mg of thallium are obtained with a relative error of 0.3% and coefficient of variation 0.46%. The effect of diverse ions are studied. The method is applied to the determination of thallium in its complexes and synthetic mixtures.