Mechanical-loss measurements were performed in three grades of martensitic high-carbon tool steels. These steels were treated under carburizing gas before quenching, in order to vary the concentration of interstitial carbon in the martensite, as assessed by thermoelectric-power measurements. The mechanical-loss spectra present a relaxation peak due to interaction between dislocations and carbon interstitials, similar to the Snoek-Koster peak found in ferrite. The amplitude of the Snoek-Koster peak is expected to be independent of the concentration of interstitials. However, the amplitude of the corresponding peak in martensite is found to increase with the concentration of interstitial carbon. This effect is attributed to an increase of the dislocation density with the carbon content. (C) 2009 Elsevier B.V. All rights reserved.