Abstract: The purification of chlorophosphonazo-mAwas reported. The dissociation of chlorophosphonazo-mAin aqueous solution was studied by spectrophtometric and potentiometric titration methods. The values of dissociation constants, pK₁=0.38, pK₂=1.12, pK₃ = 2.74, pK₄ = 6.79, pK₅ = 9.73, pK₆ = 11.46, were determined by spectrophotomeric method at identical temperature and ionic strength (20℃, μ = 0.4). Under identical conditions the results, pK₃ = 3.11, pK₄ = 6.83, pK₅ = 9.75, pK₆ = 11.52, obtained by using the potentio metric tritration method agreed well with those obtained by the spectrophoto metric method.The dissociation constants were measured by the potentio metric titration method at various temperatures. The thermodynamic parameters, ΔG°,ΔH° and ΔS°, for the dissociation process were calculated.The difference between the dissociation constants of chlorophosphonazo-mAand chlorophosphonazo-pNwas satisfactorily interpreted by the difference in the values of net atomic charge density distribution obtained with quantum chemistry calculations.