A complexometric method for the determination of mercury(II) in presence of other metal ions is described based on the selective masking action of 3-acetyl-2-thiohydantoin towards mercury(II). Mercury(II) present in a given sample solution is first complexed with an excess of EDTA and the unreacted EDTA is titrated against standard lead nitrate solution at pH 5-6 (hexamine buffer) using xylenol orange as the indicator. A 0.5% solution of 3-acetyl-2-thiohydantoin is then added to displace EDTA from the Hg(II)-EDTA complex. The released EDTA is estimated. Reproducible and accurate results are obtained for 3.96 - 80 mg of mercury(II) with relative error less than +/- 0.25% and coefficient of variation not more than 0.38%. The effects of various ions were studied. The method is Used or the analysis of mercury ill its synthetic mixtures of ions and in complexes.