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Glycerol has been shown to lower the heat denaturation temperature (T m ) of dehydrated lysozyme while elevating the T m of hydrated lysozyme (Bell, Hageman, and Muraoka, 1995. J. Pharm. Sci. 84:707–712). Here, we report an in situ elastic neutron scattering study of the effect of glycerol and hydration on the internal dynamics of lysozyme powder. Anharmonic motions associated with structural relaxation processes were not detected for dehydrated lysozyme in the temperature range of 40 to 450K. Dehydrated lysozyme was found to have the highest T m by Bell et al. (1995b). Upon the addition of glycerol or water, anharmonicity was recovered above a dynamic transition temperature (T d ), which may contribute to the reduction of T m values for dehydrated lysozyme in the presence of glycerol. The greatest degree of anharmonicity, as well as the lowest T d , was observed for lysozyme solvated with water. Hydrated lysozyme was also found to have the lowest T m by Bell et al. (1995b). In the regime above T d , larger amounts of glycerol lead to a higher rate of change in anharmonic motions as a function of temperature, rendering the material more heat labile. Below T d , where harmonic motions dominate, the addition of glycerol resulted in a lower amplitude of motions, correlating with a stabilizing effect of glycerol on the protein.