Abstract: In order to develop metal hydride technology for storing and processing tritium, the thermodynamics isotope effects of erbium dideuteride and ditritide were investigated. The pressure composition temperature (PCT) curves and activation characteristics of deuterium and tritium from erbium at different temperatures were measured. The isotherms show a wide and straight plateau, and with a unconspicuous hysteresis effect. And then relative partial molar enthalpy and entropy change for Er-D and Er-T reaction were calculated from the Van’t Hoff plots based on the measured PCT relations. In the region of atomic ratio H/Er from 0.6 up to 1.6, the enthalpy and entropy decrease with the increase of the hydrogen concentration. It can be seen that the enthalpy and entropy of ditritide are less than that of the dideuteride with the same atomic ratio, it shows slight thermodynamic isotope effects. The calculated enthalpy and entropy during the formation of the ErD₁ phase are (-199.7±7.3) kJ/mol and (-143.4±7.2) J/(mol•K), and the calculated enthalpy and entropy of ErT₁ phase are (-209.3±3.3) kJ/mol and (-152.9±3.2) J/(mol•K), respectively.