A new matrix approach is proposed to calculate the equilibrium constants of ligands substitution in a metal ion inner coordination sphere with fixed binding sites positions. The proposed method allows to reduce the number of independent variables, necessary for the titration curves fitting without deterioration in the description accuracy. The square planar complexes [MY_4-nX_n] n = 0-4 formation in solution model includes three independent variables over(K, ̄), ω_cis and ω_trans (over(K, ̄)-equilibrium constant of replacement of first ligand, ω_cis, ω_trans-parameters of mutual influence) as an alternative to four stepwise equilibrium constants and two microconstants. In the present investigation experimental spectrophotometric data published in the literature for system {Au³⁺-Cl^--Br^-} were analysed. With the help of the proposed method the equilibrium constants of chloride by bromide ion substitution in internal coordination sphere of Au(III) are calculated over(K, ̄) = 50; ω_cis = 1.3; ω_trans = 0.7.