Effects of hydrophobic environment adjusted by various alcohols on the structural stability of calfskin collagen (CSC) were studied to elucidate the nature of collagen-monomer interaction in adhesion. The stability of CSC in aqueous alcohol solutions was represented by its denaturation temperature, Td, measured by DSC. The hydrophobicity of the alcohol solutions was quantified with their specific dielectric constants, epsilon(r), calculated from their concentrations. The effect of each alcohol to stabilize or destabilize CSC was evaluated by the initial slope of each Td vs. epsilon(r) plot, denoted as -(dTd/d epsilon(r))ini and termed as stabilization power. Results showed that a hydrophobic environment with a smaller epsilon(r) lowered the stabilization power. Stabilization power ranged from -3 (strong destabilization) for phenol (epsilon(r) =12) to +0.3 (weak stabilization) for glycerol (epsilon(r)=47). In view of the encouraging results obtained in this study, the new index was therefore helpful in predicting the effects of new dental materials of known epsilon(r) values on the stability of dentinal collagen.