A valence force field for α-glycine is proposed and normal modes of vibration are discussed. The study of the small isotopic shifts of 15N and 18O is shown to be fundamental in the determination of the force field. There is a quantitative relationship between the isotopic shifts observed on low frequency absorptions and molecular conformation. The perturbations of the ion force field from one crystalline form to another and from solid state to aqueous solution are also discussed.