In order to test the concept of the physics of dissipation during first-order phase transitions in solids, we measured the internal friction (Q Ϫ1) and the relative shear modulus () during a thermoelastic martensitic transformation in a NiTi alloy. We adopted two approaches: temperature variation and application of external stress. This investigation of internal friction was carried out with various vibration frequencies , temperature variation rates Ṫ , and strain variation rates. The index l ͑coupling factor between phase interface and oscillating stress͒ and index n ͑rate exponent for the effective phase transformation driving force͒ have been calculated from the experimental data for each case and the values of l and n are about the same in the two ͑doped͒ NiTi samples, irrespective of whether the phase transition is driven by a temperature variation or stress induced process. We compare the values of n and l for the NiTi samples with that of the other samples ͑VO 2 ceramics and FeMn alloys͒, reinforcing the previous physical interpretations of these indices. We believe the indices n and l are indeed fingerprints of first-order phase transitions in solids. ͓S0163-1829͑97͒00230-0͔