Abstract: A modified spectrophotometric method for coordination nucleo-philic substitution reactions between two kinds of ligands of mono-nuclear complexes MZ#em/em#, MLj is proposed, which introduces a simpler formula for calculation and may be applied to complex systems and the hydrolysis of metal ions.As an illustration of the proposed method, we determined the spectrophotometric data of seven competitive systems in the solutions of various pH and calculated the successive stability constants and molar extinction coefficients, and the results are in good agreement with each other. The effect of the hydrolysis of metal ions on determined results depends on (1 + Σβ_a^* H^-a) and (ΣβⁱC_zⁱ)