Abstract: In a HC1 medium, amino Cacid chlorophosphorazo reacts with rare earth elements, forming β-type complexes. The color reactions arc related to the acidity of the solutions both initial and final, the kinds and concentrations of the RE.The Ce-sub group demonstrates more strong trend than the Y-sub group one in forming β-type complexes. All of this is different from the other color reagents of this kind. It is seldom seen that the REdevelops color in such high acidity.