The effect of methylurea, N, N9-dimethylurea, ethylurea, and butylurea as well as guanidine hydrochloride~GuHCl!, urea and pH on the thermal stability, structural properties, and preferential solvation changes accompanying the thermal unfolding of ribonuclease A~RNase A! has been investigated by differential scanning calorimetry~DSC!, UV, and circular dichroism~CD! spectroscopy. The results show that the thermal stability of RNase A decreases with increasing concentration of denaturants and the size of the hydrophobic group substituted on the urea molecule. From CD measurements in the near-and far-UV range, it has been observed that the tertiary structure of RNase A melts at about 3 8C lower temperature than its secondary structure, which means that the hierarchy in structural building blocks exists for RNase A even at conditions at which according to DSC and UV measurements the RNase A unfolding can be interpreted in terms of a two-state approximation. The far-UV CD spectra also show that the final denatured states of RNase A at high temperatures in the presence of different denaturants including 4.5 M GuHCl are similar to each other but different from the one obtained in 4.5 M GuHCl at 25 8C. The concentration dependence of the preferential solvation change DG 23 , expressed as the number of cosolvent molecules entering or leaving the solvation shell of the protein upon denaturation and calculated from DSC data, shows the same relative denaturation efficiency of alkylureas as other methods.