[en] Highlights: • Internal friction on a Ni–Ti-Hf High-Temperature Shape Memory Alloys. • Martensitic transformation on a Ni–Ti-Hf High-Temperature Shape Memory Alloys. • Anomalous pre-martensitic softening measured on a Ni–Ti-Hf Shape Memory Alloy. • Mechanical Spectroscopy allows measuring the Clausius-Clapeyron coefficient. -- Abstract: The martensitic transformation of Ni_49.5Ti_35.5Hf₁₅ high temperature shape memory alloy is characterized by internal friction measurements. The temperatures for the reversible B2 ↔ B19’ transformation appear in the range 410–479 K on cooling and 447–526 K on heating. In addition, a strong anomalous pre-martensitic softening of the dynamic modulus is reported in the high-temperature phase, between 850 K and 550 K, evidencing the thermoelastic character of this family of alloys. Based on the analysis of the internal friction spectra, a theoretical and experimental methodology is proposed to measure the Clausius-Clapeyron coefficient through mechanical spectroscopy. This new non-destructive method could be extremely useful for the development of advanced prototypes of shape memory alloys.