The molecular thermodynamic model studied is based on the two-state mechanism of inactivation, in which only native folded and polymorphous unfolded protein forms are present at equilibrium. The influence of solvent on protein stability is described in terms of perturbation of the protein distribution between the two conformational states. An expression derived for the chemical potential of the protein accounts for conformational changes, ideal mixing effects, and interaction of the protein with the surrounding medium. Thermal unfolding of lysozyme was then studied in the absence or presence of hydroxylic compounds. Ultraviolet difference spectroscopy was used to monitor the conformational changes induced by heating and to determine the melting temperature of the protein. The additives investigated are ethanol, glycols, and natural osmolytes. Media containing ethanol and glycols destabilized lysozyme, whereas sugars increased the conformational stability of the protein. For all of the systems examined the melting temperature was linearly related to the surface tension of the mired solvent, supporting the ability of the model to describe the influence of the solvent and composition on lysozyme unfolding. Model predictions agreed fairly well with published differential scanning calorimetric data. The influence of hydroxylic additives on protein’s conformational stability does not extend to any special property of these components, but to their ability to perturb the surface tension of water. This model can be used to interpret and correlate thermal unfolding data and to solve the problem of protein stabilization.