Thermodynamics of trivalent metal ions/hydrogen ions exchange on a fibrous cerium(IV) hydrogenphosphate have been investigated by the micro-amount uptake. Selectivities for trivalent metal ions increase in the order; La3+<Y3+<Eu3+<<Sc3+. The standard enthalpy change and entropy change of the ion exchange reaction increase in the order of Eu3+<La3+<Y3+<Sc3+and Eu3+< Sc3+<La3+<Y3+, respectively. An enthalpy - entropy compensation relation holds except for Sc3+ion. X-ray diffractometry and thermal analyses were also conducted the ion-exchanger phase for the macro-amount uptake. The maximal interlayer spacings were observed around the metal loading of 0.5mmol/g and the weight loss for the adsorbed and interlayer water at the low metal loading increased in the order; Sc3+<La3+<Eu3+<Y3+. In the case of scandium ion, the formation of insoluble scandium phosphate was confirmed by the DTA peak at 255°C and characteristic peaks to Sc(PO4) - 2H2O in the XRD pattern.