An experimental investigation was carried out on the influence of some hydroxylic additives (up to 30% by weight) on the thermal stability of erythrocyte carbonic anhydrase. The melting temperatures of the protein in the different media were determined by spectroscopic measurements. Xylitol and maltose stabilized the enzyme, whereas ethanol, 1-propanol, ethanediol and 1,2-propanediol caused a destabilization of the protein. In all the systems examined, the transition temperature was found to be strictly related to the surface tension of the medium. A molecular thermodynamic model was developed, based on the chemical equilibrium between the native and denatured protein forms, which enabled to interpret the observed behaviour.