Abstract: Isobestic point spectrophotometric method is proposed to determine the compositions and stability constants of complexes in a coexistent system of mononuclear and polynuclear complexes. This method has been used in a indium-xylenol orange system, the compositions of the two complexes obtained are 1:1 (In:XO,λ_max=510 nm) and 2:1(λ_max=560 nm), and the stability constants are K₁ = 8. 83×10⁸ and K₂=1. 44×10⁷, respectively.

Key words: Complex composition, Complex stability constant, Indium-xylenol orange complex