Insulinotropin (glucagon-like peptide I) is a peptide containing 31 amino acid residues. It stimulates the secretion of the hormone insulin. The solubility of this peptide is highly dependent on its environment and the treatment that it has undergone. For instance, synthetic insulinotropin is highly soluble in neutral phosphate-buffered saline (1 mg/mL). However, the application of shear force by stirring renders it extremely insoluble (1 micrograms/mL). This property may be explained in terms of a change in peptide secondary structure with no alteration in primary structure. In order to understand this phenomenon, FT-IR and near-IR FT-Raman were employed to examine four samples prepared under different experimental conditions. It was found that solubility decreases as the alpha-helix is converted to an antiparallel beta-sheet structure.