Abstract: In this paper, ten carboxyazo-type reagents( Ⅰ ) have been designed and. synthesized. Their colour reactions with rare earth elements (Ⅱ) were investigated.1. The sensitivities of colour reactions of ( Ⅰ ) with (Ⅱ) decrease with increasing electronegativities of auxochrome groups.2. The effect of positions of auxochrome group on the sensitivities of colour reactions between (Ⅰ) and (Ⅱ) and of bathochromic effect of colour-complexes is generally accordance with the following rules.The auxochrome group placed in the para-position of azo group (-N=N-) is greater than in the meta-position.3. When -COOH group is placed in the ortho-position of azo group,(Ⅰ) (Ⅰ) give the colour reactions to (Ⅱ)in grouping;(2)the sensiti-vities of colour reactions and stabilities of colour-complexes decrease as the atomic numbers of (Ⅱ) increase; (3) co-colour reaction and (4) acid effect were observed. When -COOH group is placed in the meta-or para-position of azo group, (Ⅰ) can be used to determine the total amount of ( Ⅱ ).The authors consider that the steric effect of molecules of the reagents, the variation of ionic radius of (Ⅱ) resulting from lantha-nide contraction and the ability of proton releasing of chelating group are reasons for the above effects, (2) and (4), in 3, It was determined by the equilibrium shift method that the comopsition of the complex is.La:p-Br-3C = 1:3 where p-Br-3C is p-Br-3-carboxyazochromotropic acid.