The change in Gibbs free energy resulting from changes in the temperature (T-t) and heat (DeltaH(t)) of an inverse temperature transition (ITT) of hydrophobic association is derived and utilized to develop a hydrophobicity scale for amino acid residues within an elastic model protein, (GVGVP)(n). On raising the temperature, hydrophobic residues go from water to hydrophobic association. Using T-t and DeltaH(t) of the ITT, the change in Gibbs free energy for hydrophobic association, DeltaG(HA)(0), in kcal/mol-(GXGVP) where X is the guest amino acid residue, was calculated for the naturally occurring amino acid residues in PBS. DeltaG(HA)(0) varies by more than 10 kcal/mol-(GXGVP) from the most hydrophobic tryptophan residue to the most hydrophilic glutamate residue. (C) 2004 Elsevier B.V. All rights reserved.