Abstract: The relation between the auxochromes, solvents and a-type, β-type reaction was investigated in this paper. It shows that only the bisazo derivaties of chromotropic acid containing stronger electrophilic auxochromes such as -Cl, -NO₂, -COCH₃, -COOH can form β-type chelates with rare earth elements, and the auxochromes of -SO₃H, -PO₃H,-AsO₃H₂ only form in a-type.This paper points out that under proper conditions the bisazo derivatives which form β-type chelates can be extracted by isoamyl alcohol, but not the a-type.The formation of β-type chelates can be inhibited by some solvents such as ethyl alcohol and acetone. But these solvents can greatly raise the sensitivity of the a-type reaction.