Abstract: p-acetyl-arsenazo is a new color reagent for spectrophotometry determination of rare earths. In the medium of HCl-NaAc, it forms α-type of complexes with Ce-group rare earths. It is highly sensitive and selective: ^ε670=(10.3-11.6)×10⁴(La-Nd).The complexes are very stable. The Beer's law is obeyed from(0-70)μg/25ml (La-Nd, oxide). This reagent has been used to determine micro Ce-group rare earths in alloy steel by direct spectrophotometry.Under a certain experimental condition, the a-type of rare earths complexes (La-Nd) can be converted to β-type, λ_max = 730nm. The tendency of the conversion of complexes decreases as the atomic number increases, which can enlarge the difference between Ce-group rare earths. The a-type complexes converted to β-type can be influenced by the reaction medium, the acidity, the molar ration of [RE]/[R], the standing time,the organic solvent and others.In this paper the color reactions of two types of rare earths with p-acetyl-arsenazo and the conditions of conversion have been studied. The composition of complexes of two types have been determined.