A new method for determination of the compositions of rare earth compl
exes has been developed based on high performance liquid chromatograph
y (HPLC). When rare earth ion forms stable complex with ligand, the ex
cess of ligand can be measured by HPLC and used to evaluate the compos
ition ratio of the complex. This method has been applied for the syste
ms of rare earths with xylenol orange (XO) and with chlorophosphonazo
I(CPAI).