We have determined the partial molar volumes, expansibilities, and adiabatic compressibilities for poly(L-alanine), poly(L-proline), and poly(L-threonine) within the temperature range of 18 -55°C. In addition, we have determined at 25°C changes in volume, ⌬V, and adiabatic compressibility, ⌬K S , associated with the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) and the ␣-to-␤ transition of poly(L-lysine). We have interpreted our volumetric data as suggesting that poly(L-alanine) and poly(L-proline) are not fully unfolded and, probably, retain some solvent-inaccessible core. Further, we propose that poly(L-threonine) is fully unfolded with the majority of its atomic groups being solvent-exposed. Near zero changes in volume and compressibility accompanying the coil-to-helix transitions of poly(L-lysine) and poly(L-glutamic acid) suggest that, in the absence of fortuitous compensations, the coil-to-helix transitions of the polypeptides do not result in any significant enhancement of solute hydration. By contrast, the ␣-to-␤ transition of poly(L-lysine) causes slight but statistically significant increases in volume and compressibility, an observation that may suggest that the ␤-sheet conformation of poly(L-lysine) is slightly less hydrated than its