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Thermal denaturation curves of lysozyme and ribonuclease-A were determined by measuring their far-UV circular dichroism (CD) spectra in the presence of different concentrations of five polyols (sorbitol, glycerol, mannitol, xylitol and adonitol) at various pH values in the range 7.0–1.9. The denaturation curve at each polyol concentration and pH was analysed to obtain values of T_m (midpoint of denaturation) and ΔH_m (enthalpy change at T_m), and these ΔH_m and T_m values obtained at different pH values were used to obtain ΔC_p (constant-pressure heat capacity change) at each polyol concentration. Using values of ΔH_m, T_m and ΔC_p in the Gibbs–Helmholtz equation, ΔG_D° (Gibbs energy change at 25 °C) was determined at a given pH and polyol concentration. Main conclusions of this study are that polyols have no significant effect on ΔG_D° at pH 7.0, and they stabilise proteins in terms of ΔG_D° against heat …