betaB2- and gammaC-crystallins belong to the betagamma-crystallin superfamily and have very similar structures. Molecular spectroscopic techniques such as UV-visible absorption, circular dichroism, and fluorescence indicate they have similar biophysical properties. Their structures are characterized by the presence of two domains consisting of four Greek key motifs. The only difference is the connecting peptide of the two domains, which is flexible in gamma-crystallin but extended in beta-crystallin; thus, an intradomain association and a monomer are formed in gamma-crystallin and an interdomain association and a dimer are formed in beta-crystallin. The difference may be reflected in the thermodynamic stability. In the present study, we calculated the standard free-energy by equilibrium unfolding transition in guanidine hydrochloride using three spectroscopic parameters: absorbance at 235nm, Trp fluorescence intensity at 320nm, and far-UV circular dichroism at 223nm. Global analyses indicate that both dimeric betaB2- and monomeric gammaC-crystallins are a better fit to a three-state model than to a two-state model. In terms of standard free-energy, deltaG(0)(H(2)O,i) both betaB2-crystallin and gammaC-crystallin are stable proteins and dimeric betaB2-crystallin is more stable than the monomeric gammaC-crystallin. The significance of the thermodynamic stability for betaB2- and gammaC-crystallins may be related to their functions in the lens.