Abstract: The color reactions of α-type and β-type of rare earths with chloro-phosphoaazo Ⅲ have been studied in this paper.Under a certain experimental condition,a-type of heavier rare earths (Gd to Lu) chelates of chlorophosphonazo Ⅲ can be converted to β-type.The tendency of the conversion of chelates of yttrium is the largest.The corresponding chelates of lighter rare earths (La to Eu) do not convert to β-type under the same condition.Masking effect of Zn-EDTA and Zn-CyDTA can be utilized to enlarge the difference of color reactions.Amethod for spectrophotome-tric determination of lighter and heavier rare earths,and yttrium in HCl-NaAc buffer solution has been suggested.Lighter rare earths can be determined in the presence of heavier rare earths at 684nm,pH 2.9-3.4,using Zn-CyDTAas the masking agent.Heavier rare earths and yttrium can be determined respectively in the presence of lighter rare earths at 740nm in tke range of pH2.8-3.6 and pH1.8-2.4,using Zn-EDTA+ NaFa nd NaF as a masking agent respectively.