Based on a non-equilibrium kinetic extraction technique, the complexation kinetics of diethylenetriamine- pentaacetic acid (DTPA) with rare earth (RE) ions was investigated with different adding sequences of DTPA. The results indicated that the separation factors of β_Tm/Er and β_Y/Er were higher when adding DTPA at the start of extraction than those before extraction. The extraction order for Y, Er and Tm was Tm>Y>Er. The root of discrepancy in complexation kinetics of Y, Er and Tm ions with DTPA and the enhanced kinetic separation mechanism were elucidated from the forward complex formation and reverse dissociation rates by the stopped-flow spectrophotometric technique. The apparent complexation rate constants began to decrease gradually with the increase of aqueous pH, while increased with increasing DTPA concentration. The emergence of extraction priority order as Tm>Y>Er was verified according to the calculated reaction kinetic constants of Y, Er and Tm at different pH values.