Abstract: The boundary of steady-state zones was investigated from the point of view of t he-equilibrium theory and the rate theory when the rare earths were separated with NTA, by means of high pressure ion-exchange chromatography. It has been ascertained that the mass-transfer process is mainly controlled by film diffusion, and longitudinal diffusion may actually be neglected under the condition of high flow rate and HETP is not affected by retaining ion or concentration of displacing ion.The experimental results show that when the flow rate of displacer increases from 2.5 cm/min to 25 cm/min, the plot of HETP with respect to flow rate is linear, the slope and intercept of the straight line are related to the properties of the ion to be separated and HETP will only increase twice when the linear flow rate is increased one order of magnitude. These results show that NTA is excellent in kinetic properties.