At this thesis, an on-line system for preconcentration and separation of lead(II) and cadmium(II) is presented. The method is based on the complex formation of Pb(II) and Cd(II) with adsorbed Methyl thymol blue on activated carbon. The conditions of preparing the solid phase reagent and of quantitative recovery of Pb(II) and Cd(II) from diluted solutions, such as acidity of aqueous phase, solid phase capacity, and flow variables were studied as well as effect of potential interfering ions. After preconcentration step, the metal ions were eluted automatically by 5 ml of 0.5 M HNO 3 solution and the metal ions content was determined by flame atomic absorption spectrometry. Under the optimum conditions, the lead ions in aqueous samples were separated and preconcentrated about 1000-fold by the column. The detection limit was 0.001 µg mL -1 . The relative standard deviation for ten independent determinations reached values of 4.2, 4.4 and 4.3% for lead solutions of 0.2, 1.0 and 5.0 µg mL -1 , respectively. Lead has been determined in river and tap water samples, with recoveries of 98 to 102%. Also, the method can be used for preconcentration of cadmium(II) down to 0.001 µg mL -1 . Under the optimum conditions, cadmium in aqueous samples was concentrated about 1000-fold. The relative standard deviations for ten independent determinations reached values of 4.2, 4.0 and 3.0% for cadmium solutions of 20, 50 and 100 ng mL -1 Cd(II), respectively. Cadmium has been determined in river and tap water samples, with recoveries of 97 to 101%.