The melting point of &egr;‐carbobenzoxy‐&agr;‐carboxyl‐l‐lysine anhydride stored at room temperature for one year (I) rises considerably above that of the freshly prepared compound (II) (m.p. 100°). X‐ray diffraction studies indicate: (II) is probably monoclinic,a≃90A,b=5A,c≃10A; (I) suffered disorientation giving diagrams similar to those of bulk polymer (III); threshold conditions for the disorientation process are heat‐treatment of (II) at 70° for 24 hours; the change appears to be complete when (II) is heat‐treated at 85° for 24 hours since the diagrams are similar to those of (III) and no further change is observed in the diagrams of samples treated under more severe conditions below the melting point. The largest Bragg spacing of 29A observed for (II) is of the same order of magnitude as the over‐all length of a fully extended hydrogen‐bonded dimer of (II). The smallest refractive index of (II) is parallel to thebaxis. The structure of (II) is concluded to be a triple dimer layer arrangement which would involve a relatively small entropy change on polymerizing in the solid state to give (I) and (III).