Abstract: This paper deals preliminarily with the substitution reaction mechanism of RE(DBC)₃(DBC = 2-(2-arsenophenylazo)-7-(2, 6-dibromo-4-chlo-rophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid) with CyDTAbased on the experimental facts and theory deduction. Akinetic reaction system for simultaneous determination of Sm and Gd was established by choosing the best substituent and complexing agent from several aminocarboxylic acids and polyhalidarsenazoes. Under the suitable conditions Sm and Gd in their concentrated material were determined simultaneously in this kinetic reaction system. The result agrees satisfactorily with that obtained by JCP-AES. The synthetic samples of Sm and Gd mixture with a content ratio in the range of 1:2～4: 1(Sm:Gd) were determined by this method. The error of it is about 10%. It is predictable that the method would be used for a control of procedure in the rare earth production.

Key words: Simultaneous determination of Sm and Gd, Kinetic method of rare earth, Rate differential determination of single rare earth