Abstract: The extraction behavior of Cu(Ⅱ),La(Ⅲ),U(Ⅵ) and Ce(Ⅳ) in the bi-aqueous phase liquid-liquid extraction system of PEG-(NH4)2SO4-Zincon,especially the effects of acidity,the concentration of zincon,the amount of (NH4)2SO4,and the presence or absence of surfactants,were studied.Cupric ion,in the form of a chelate with zincon was found to be quantitatively extracted into the PEG phase in the pH range of 3-8,giving the extraction rate over 95%.Lanthanum(Ⅲ) was not extracted and remained as free ion in the lower aqueous phase in the pH range of 1 to 7.Incomplete extractions of U(Ⅵ) and Ce(Ⅳ) were observed in the range of pH 1-11,giving the extraction rates between 30%-65%.It was shown that at pH 6,Cu(Ⅱ) was separated quantitatively from co-existing La(Ⅲ),giving extraction rates in the range from 95.6%-98.3%,while the extraction rates of La(Ⅲ) were in the range from 4.5% to 5.1%.The reaction mechanism of the metal ions with zincon as a chelating agent during the course of extraction was also studied priliminarily.