A selective method for the extraction of mercury(II) from hydrochloric acid solution with 3,4,4a,5-tetrahydro-3,3,4a-trimethyl-7-(substituted)-pyrimido [1,6-a]-benzimidazole-1-thiol, (PBT) is proposed. The extraction ability of the system was studied both in strongly acidic (6 mol dm–3 HCl) and alkaline (pH 12) media. The optimum extraction conditions were evaluated from a critical study of the effect of pH, PBT concentration and metal ion concentration. The probable composition of the species has been deduced from lg-lg plots and an extraction mechanism is suggested. A separation scheme is proposed. The method has been applied for the determination of Hg (II) in industrial waste water.