Abstract: As is well known,the mechanism of La³⁺- NaY ion-exchange reaction reviewed by H.S.Sherry has been mainly ascribed into two steps.The second step of them called the slow step is proposed as the rate-controlling step in the whole reaction with respect to the stripping of water molecules from the hydrated La³⁺ion in the large cavity in relation with the replacement of Na⁺by La³⁺in the β-cage.The corresponding activation energy can be estimated by the enthaly of hydration of La³⁺ion.Although the assumption of the rate-controlling step is reasonable,there is still lack of exact experimental evidence.It is to make a deeper understanding of the slow step by means of experimental devices.The arrangement of measurement condition at elevated temperature may facilitate the systematical study of the behaviour of this kind of ion-exchange reaction.The present authors have investigated the slow step at an elevated temperature in three aspects;1.To measure the ion-exchange isotherm,for La³⁺-NaY system at 100℃and 180℃ respectively.2.To investigate the degree of ion-exchange of individual LnYat 180℃ and to compare the results with the enthalpy of hydration of Ln³⁺ions.3.To evalute the rate-constant and apparent activation energy of the slow step at elevated temperature from our experimental data.As a result,a more rational mechanism of La³⁺-NaYion-exchange reaction is proposed.Ⅰ.La(H₃O)₉³⁺+ NaY(s)→La(H₂O)₉³⁺Na(SI)F(s)+Na⁺Ⅱ.La(H₂O)₉³⁺(in large cavity)+La(H₂O)₉³⁺Na (SI)Y(s)→La(H₂O)₉³⁺LaY(s)+Na⁺Eq-Ⅱ should consist of the following two consecutive reactions;Ⅱ-A.La(H₂O)₉³⁺(in large cavity)→La³⁺+9H₂OⅡ-B.La³⁺+ La (H₃O)₉³⁺Na(SI)F(s)→La(H₂O)³⁺LaY(s)+Na⁺Eq-Ⅱ-Ais the rate-controlling step for this ion exchange reaction.