The lower critical solution temperature (LCST) of elastin-like polypeptides (ELPs) was investigated as a function of ELP chain length and guest residue chemistry. These measurements were made in both D2O and H2O. Differences in the LCST values with heavy and light water were correlated with secondary structure formation of the polypeptide chains. Such structural information was obtained by circular dichroism and infrared measurements. Additional thermodynamic data were obtained by differential scanning calorimetry. It was found that there is a greater change in the LCST value between H2O and D2O for those polypeptides which form the greatest amount of beta-turn/beta-aggregate structure. Moreover, these same molecules were. the least hydrophobic ELPs. Therefore, hydrogen bonding rather than, hydrophobicity was the key factor in the stabilization of the collapsed state of ELPs in D2O compared with H2O.