The introduction of highly fluorinated analogues of hydrophobic amino acid residues into proteins has proved an effective and general strategy for increasing protein stability toward both chemical denaturants and heat. However, the thermodynamic basis for this stabilizing effect, whether enthalpic or entropic in nature, has not been extensively investigated. Here we describe studies in which the values of Delta H degrees, Delta S degrees, and Delta C-p degrees have been determined for the unfolding of a series of 12 small, de novo-designed proteins in which the hydrophobic core is packed with various combinations of fluorinated and non-fluorinated amino acid residues. The increase in the free energy of unfolding with increasing fluorine content is associated with increasingly unfavorable entropies of unfolding and correlates well with calculated changes in apolar solvent-accessible surface area. Delta C-p degrees for unfolding is positive for all the proteins and, similarly, correlates with changes in apolar solvent-accessible surface area Delta H degrees for unfolding shows no correlation with either fluorine content or changes in apolar solvent-accessible surface area. We conclude that conventional hydrophobic effects adequately explain the enhanced stabilities of most highly fluorinated proteins. The extremely high thermal stability of these proteins results, in part, from their very low per-residue Delta C-p degrees, as has been observed for natural thermostable proteins.