Statistics for A simple and selective EDTA method using a masking and demasking technique is proposed for the determination of thallium(III). The thallium is complexed with excess of EDTA, the surplus being back-titrated (pH 5-6, hexamine buffer) with zinc sulphate solution (Xylenol Orange as indicator). 4-Amino-5-mercapto-3-propyl-1,2,4-triazole is then added and the mixture heated on a water-bath for 5-10 min to displace EDTA from its thallium complex. The EDTA liberated is titrated with zinc sulphate solution. Reproducible and accurate results are obtained in the range 5-75 mg of thallium with both the relative error and coefficient of variation not exceeding 0.4%. © 1987.