Bjerrum's method for determining the values of successive constants in step equilibria has been shown to be unsuitable in cases where the ligand is charged. Suitable modifications have been introduced to adapt Bjerrum's method in cases of charged ligand, taking into account the activity coefficients of the ions present in the solution, calculated by the application of the "Debye-Huckel limiting law" from the value of the ionic strength of the solution mixture. The phs of aqueous solution mixtures of lead acetate and nitric acid at different concentrations after attainment of equilibrium have been determined, and the data utilised to calculate the thermo­dynamic dissociation constants, K1 and K2, for the two successive equilibria, PbA2\(\rightleftharpoons\) PeA+ + A- and PbA+ \(\rightleftharpoons\) Pb2++A- (where, A stands for acetate or propionate ion), applying Bjerrum's method modified as above. The inapplicability of the Delve-Hứckel limiting law to electrolyte solutions at finite concentrations has also been properly considered, and ingenious corrections have been made in finding the value of K1 and K2, which are 31.2 X 10-3 and 3.31 X 10-3 at 31-34.5° respectively for lead acetate and 5.11 x 10-3 and 4 57 x 10-3 at room temperature (21-30°) for lead propionate.