Abstract: We have studied the kinetics of the complex formation reaction between rare earths and dibromochloroarsenazo, and presented a differential kinetic method for simultaneous determination of gadolinium and dysprosium in a mixture.The reaction rate constants of rare earths decrease with increasing their atomic numbers. A "Gd broken" phenomenon was found for the first time. The kinetics equations of Gd and Dy were deduced, and that the active energies of Gd and Dy are 29.216 kJand 53.35 kJrespectively was reported. Linear regression and graphical extrapolation appraochs have been applied to treatment of the kinetic analysis data. The comprision of the two methods shows that linear regression appra-och is better for the Gc³⁺/Dy³⁺-Dibromochloroarsenazo syslem, which has been used to determine Gd and Dy in a synthetical n ixture sample down to 10^-7M with accuracy of 0 to 13.5%.