Internal friction measurements were made at 1 Hz in the temperature range of 25 to 500°C on quenched, tempered and cold-worked Fe-Ni-C martensites. The alloys, which contained 15 to 30 wt pct Ni and 0.1 to 1.0 wt pct C, andM s temperatures <0°C, transformed to martensite with a twinned (259)γ habit and exhibited a relaxation peak at ~160°C. These results could be contrasted with those for Fe-C martensites, which form above room temperature, have predominantly dislocated (111)γ or (225)γ habits, and exhibit an internal friction peak at about 250°C. The literature on substructures, tempering and internal friction of all ferrous martensites and cold-worked ferrites was utilized in the interpretation of the 160°C peak. Several dissimilarities between the 160°C peak and the 250°C peak in Fe-C martensites or the cold-work peak in ferrite were noted such that models of dislocation-interstitial interaction for these peaks could not explain the 160°C peak. It was concluded that the 160°C peak is associated with the stress-induced motion of twin boundaries containing mobile carbon atoms. Such a mechanism was shown to be consistent with the present experimental observations and all other available data.