The heat capacity of polycrystalline Ce 0.8 Y 0.2 O 1.9 and of its water/hydrogen-dissolved sample was measured from 6 to 300K using an adiabatic calorimeter. The heat capacity of the water/hydrogen-dissolved sample is larger than that of the dry sample below 50K, and the difference is about 10% at 10K. Such a difference should originate in the change of lattice vibration spectra caused by the introduction of water/hydrogen. The mechanism is discussed in terms of the defect structure.