Abstract: The solutions containing equimolar thorium nitrate,EDTA and an amino-acid (serine,threonine,leucine,or a-amino-isobutyric acid) have been investigated potentiometrically.The results indicate the formation of 1:1:1 mixed ligand chelates.The formation constants logK_ThYL^ThY at 30°±0.1℃ and ionic strength μ=0.15 (KNO₃) have been calculated.The order of stabilities in terms of the secondary ligand amino-acids,is as follows.a-amino-isobuturic>leucine>threonine=serine.This order agrees with that of the basicity of the amino-acids and can be represented by a linear equation