Abstract The heat capacity of deuterated ammonium hexafluorophosphate ND 4PF 6 was measured from 5.8 K to 347 K by adiabatic calorimetry. Two λ-shaped anomalies were found in the curve for heat capacity against temperature. The upper anomaly reached a peak C p,m ≈ 600· R at (194.4 ± 0.05) K with Δ trs S m o = (1.013 ± 0.016)· R ( R = 8.3145J·K −1·mol −1). The lower anomaly is also λ-shaped but is gradual, continuous, and reaches its maximum C p,m = 19.2· R at (137.5 ± 1.0) K with Δ trs S m o = (0.562 ± 0.011)· R. Both anomalies are characteristic of order-disorder transitions. Smoothed values of the standard thermodynamic quantities are tabulated up to 350 K.