On the basic theory of extration kinetics, the rule of transition of extration kinetics behavior of rare earth elements with 2-Et hexyl 2-Et hexyl phosphonic acid [HEH(EHP), P507] has been studied using stirred Lewis' cell. The transformation rules of distribution ratio and separation factor of rare earth elements with time are discussed. The forward rate constant (kf) and the backward rate constant (kb) are measured. The "tetrad effect", the "oblique W effect" and the "gadolinium break" phenomenon of the rate constants have given for the first time in extration kinetics.

Key words: HYDROCHLORIC ACID, RARE ELEMENT, RATE CONSTANTS, EXTRACTION KINETICS