To gain insight into the thermal stability of intermediate filaments and matrix in the biological composite structure of alpha-keratins, the thermal denaturation performance of human hair fibers was investigated by Modulated Differential Scanning Calorimetry (MDSC) in the dry and the wet state. Denaturation enthalpy Delta H-D in water was found to be independent of heating rate (11.5 J/g) and to be approximately double as high as in the dry state (5.2 J/g). The lower enthalpy (dry) and its dependency on heating rate are attributed to effects of pyrolysis. The stepwise change of reversing heat capacity Delta C-p marks the denaturation process as a classic two-stage transition. The increase of Delta C-p with heating rate reflects a continuous shift of the nature of the denaturation of the alpha-helical material, first, into random coil and then towards random beta-materials for lower heating rates. Denaturation temperatures follow Arrhenius relationships with heating rate, yielding activation energies of 416 kJ/mol (dry) and 263 kJ/mol (wet), respectively. A decrease of activation energy (wet) for high heating rates supports the hypothesis of systematic changes of the pathway of denaturation. (C) 2011 Elsevier Inc. All rights reserved.