The complex of lead (II) with the reagent 3-methyl-1, 2-cyclopentanodione dithiosemicarbazone, 3-Me-CPDT, has been studied. The composition, stability constant, and free energy change of formation of the complex have been determined. A sensitive spectrophotometric method for the determination of lead in aqueous solutions has been developed and applied for a range of concentration of 0–60μg/ml of lead using the complex Pb-3-Me-CPDT. The optimum conditions for maximum sensitivity of analysis such as standing time, pH, and wavelength have been determined. The effect of foreign ions on this method has also been studied.