The terahertz dielectric response of partially thermally denatured, hen egg white lysozyme (HEWL) films is measured as a function of frequency and hydration using terahertz time domain spectroscopy (THz-TDS). Results are compared to similar measurements on native state samples. The frequency and hydration dependence of the absorbance for the two sample types are highly similar except for a notable suppression at ~ 0.4 THz (13 cm-1) in the partially denatured sample. In contrast to the native state sample which has a nearly frequency independent index of refraction, the index of the partially denatured sample decreases as a function of frequency. A transition is observed in both the absorbance and the index at a hydration level of ~ 0.25h (grams H2O per gram lysozyme). Below the transition, the response slowly increases while above 0.25h, the slope of both the absorbance and index sharply increases. Interestingly, we observed similar behavior in the native sample. The Cole-Cole plots exhibit a hydration dependence that is distinct from the native sample and indicative of neither pure resonance nor dielectric relaxation. We consider the implications of these results on THz biomolecular sensors.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.