Abstract: In the presence of the activator ethylenediamine, the catalytic effect of Ag⁺on the coloration reaction based on the oxidation of reduced phenolphthalein with (NH₄)₂S₂O₈was studied. Anew catalytic-kineitc spectrophotometric method for determining Ag was developed and its optimum conditions were established. satisfactory result was obtained when the method was applied to trace analysis of Ag in water and waste water.