Abstract: Chlorophosphonazo-mA [2-(4-Chloro-2-phosphonobenzenenazo)-7-(3-ac-etylbenzenenazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid] is a derivative of unsyr.metrical bisazo chromotropic acid,with the following structure In 1979,it was first reported that the title compound could be used in the spectro determination of the total amount of micro rare-earths existing in spherical graphite cast iron and weak alloy steel and direct spectro determination of the total amount of the rare-earths and the fractional amount of the light and heavy rare-earths when used together with chlophosphonazo-mN It is a rather ideal chromogenic reagent to determine the total amount of the rare-earths.It was also discovered in later work to be usefulin the spectro determination of the elements uranium(VI)and calciumetc. In order to make a further study for the thermodynamic properties and kinetic characteristics of the reagent when complexing with the rare-earths we have first investigated the ionization of the chlorophosphonazo-mAin solution.This paper reports the results of determining the successive ionization constants and the thermodynamic parameters of each step of chlorophosphonazo-mAby pHpotentiometric and spectrophotometric methods.