Abstract: Trace amount of thallium in high-purity indium was separated from the matrix by cation exchange and determined by GF-AAS.The indium sample was dissolved in nitric acid.The strongly acidic cation exchange resin of model 732 was used in the separation.It was shown by experiment,that macro amounts of indium,together with traces of ions of As,Al,Fe and Sn were separated from thallium(Ⅲ);1.0 μg of Pb,Si,0.5 μg of Zn,Cd,Mg,Ag,Ni and 0.4 μg Cu,were found to be not interfering with the determination of 0.2 ng thallium.Detection limit of the method obtained was 6 pg.Test for recovery was made by standard addition method and values of recovery were in the range from 94% to 108%.