Summary FT-IR spectroscopic studies of dynorphin A(1–13) in H 2O and D 2O are utilized to derive the aqueous phase secondary structure of the opioid peptide. Resolution enhancement of the amide I region of dynorphin A(1–13) in H 2O revealed a doublet at 1652 cm −1 and 1669 cm −1 which are interpreted as indicative of “unordered” and extended structures. From FT-IR and 1H NMR deuterium exchange studies, the peptide NH groups appeared to be solvent accessible which is suggestive of an essentially extended structure with aperiodically interwoven “unordered” structure. The results are consistent with Raman Spectroscopic (Rapaka et al., (1987) Int. J. Peptide Protein Res. 30:284–287) and 2D NMR studies (Huang et al. submitted), from our laboratory.