We investigated the assignments and the conformational dependencies of the UV resonance Raman bands of the 21-residue mainly alanine peptide (AP) and its isotopically substituted derivatives in both their alpha-helical and PPII states. We also examined smaller peptides to correlate conformation, hydrogen bonding, and structure. Our vibrational mode analysis confirms the complex nature of the amide III region, which contains many vibrational modes. We assign these bands by interpreting the isotopically induced frequency shifts and the conformational sensitivity of these bands and their temperature dependence. Our assignments of the amide bands in some cases agree, but in other cases challenge previous assignments by Lee and Krimm (Biopolymers 1998, 46, 283-317), Overman and Thomas (Biochemistry 1998, 37, 5654-5665), and Diem et al. (J. Phys. Chem. 1992, 96, 548-554). We see evidence for the partial dehydration of alpha-helices at elevated temperatures.