The heat capacity of the orthorhombic salt: deuterated ammonium perchlorate, ND4 CI0 4 , was measured from 7 to 345 K using adiabatic calorimetry. The heat capacity against temperature curve is smooth, continuous and without anomaly. Values of the standard molar thermodynamic quantities are presented up to 340 K. The heat capacities ofND4 CI0 4 and NH4 CI0 4 have been analyzed. The contributions to the vibrational heat capacity from the external optical modes ofNH/ or ND 4 +, CI0 4and libration

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... from the external modes of CI0 4 -along with those of vibration from the internal optical modes ofNH/ or ND4+ and CI0 4 -, and the acoustic lattice modes for these ions have been calculated. The difference between the experimental and calculated heat capacity, called the residual heat capacity, equals the contribution from ammonium ion rotation and the thermal expansion of the lattice. With recent thermal expansion data, the correction from constant stress to constant strain has been applied and the derived rotational heat capacities of the NH/ and ND/ are determined to be in qualitative agreement with those derived from various rotational models.