Abstract: In this paper, two series of arsenazo types reagents with diffrent auxochromic groups and positions for rare earth, thorium etc. have been designed and synthesized. The reagents were purified by TLCand their strutures were determined by infrared spectra, quantitative elemenitary analysis and thermogravimetrici analysis. They are easily soluble in water, alcohol and acetone but insoluble in benzene and ether. The colour reaction of the reagents with rare earth, thorium and zirconium elemnts have also been studied. The result shows that the effects of their molecular structure on the colour reaction have some regularity. The absorption maxima of the complexes of reagents o-As-TX (X=C₁, Br,I) with La, Gd, Yb gradually increase with the dercrease in electronegativity of subtituent Gl, Br, I, but with Y, the phenomenon is contrary. The sensitivity of o-As-DXN reacting with rare earth elements are lower and rapidly decreases with the increase in acidity of medium, but with Th, Zr, it is still higher. In these colour reagents, o-As-DBNis excellent colour reagent in the determination of thorium The tolerant quantity of rare earth elements can reach several milligrams, especially for uranium.