Thermal denaturation of hydrated keratin in wool was investigated by NMR using H-1 wide-line spectra to obtain the phase composition and H-1 spin-diffusion experiments using a double-quantum filter to obtain the domain sizes for the wool fibers. The denaturation process detected by DSC takes place for wool fibers in deuterated water in the temperature range 140-144 degrees C. The phase composition measured by H-1 wide line NMR spectra reveals a rigid, semirigid and an amorphous phase for temperatures in the range 25-160 degrees C. A dramatic change in the phase composition was detected around 142 C, corresponding to the denaturation temperature. The morphological domain sizes measured by H-1 spin-diffusion NMR experiments were obtain from the solutions of the spin-diffusion equations for two-dimensional rectangular and cylindrical morphologies. The keratin mobility gradient in the interfacial region at different denaturation temperatures was measured from the H-1 spin-diffusion data. A qualitative model describing the denaturation process of hydrated keratin protein was developed that explains the changes in domain thickness, spin diffusivities, phase composition, and thermodynamic parameters.