The protein unfolding process observed in a narrow temperature range was clearly explained by evaluating the small difference in the enthalpy of hydrogen-bonding between amino acid residues and the hydration of amino acid residue separately. In aqueous solutions, the effect of cosolute on the protein stability is primarily dependent on water activity, a(w), the role of which has been long neglected in the literature. The effect of a(w), on protein stability works as a power law so that a small change in a(w), is amplified substantially through the cooperative hydration effect. In the present approach, the role of hydrophobic interaction stands behind. This affects protein stability indirectly through the change in solution structure caused by the existence of cosolute. (C) 2016, The Society for Biotechnology, Japan. All rights reserved.