Abstract: Protonation of chlorophosphonazo-m-NO₂ takes place in acidic medium (Ig K_p = -2.11) while dissociation of the first hydroxyl group from naphthalene ring is shifted toward the basic region (pK_α= 10.21) .Various complexes of La-CPAmN are formed in the pHrange of 1.5-2.0. The mole ratios La/CPAmN of these complexes have been estimated to be 1:1, 1:2 and 1:3 by Job's continuous variation method and the equilibrium shift method. The β-type complex (1:1) giving a characteristic maximum absorption at 715 nm is formed under a certain experimental condition.The 1:3 complex existed in the excess of reagent is very stable in acidic medium with an overall stability constant lg K_s, of 45.40.