A simple and selective complexometric method for the determination of thallium in presence of other metal ions is proposed on the basis of selective masking ability of thiomalic acid towards thallium(III). Thallium present in a given sample solution is first complexed with a known excess of EDTA and the surplus EDTA is titrated with standard lead nitrate solution at pH 5-6, using xylenol orange as the indicator. A 0.6% aqueous solution of thiomalic acid is then added to displace EDTA from the TI(III)-EDTA complex. The released EDTA is titrated with standard lead nitrate solution. Reproducible and accurate results are obtained for 3-92 mg of TI(III) with relative error less than +/- 0.17% and coefficient of variation not more than 0.36%. The interference of various ions is studied. This method is used for the analysis of thallium in its synthetic alloy mixtures and also in complexes.