The high-resolution structure of an α-chitin/hydrazine complex has been determined to reveal the molecular interactions between hydrazine molecules and chitin chains. The complexation with guest hydrazine molecules improves the crystallinity of α-chitin from the original form, so that the primary hydroxyl group is clearly located in the gt position on the chitin chains arranged in an antiparallel manner in contrast to the original α-chitin structure which has the disorder between gt and gg positions. During the complexation process, α-chitin chains translate by c/4 to incorporate hydrazine molecules. Molecular dynamics calculations based on the refined structure show that hydrazine nitrogen atoms are disordered with competing hydrogen bonds. These observations suggest a mechanism for the structural conversion of polysaccharides to their crystal solvate forms during penetration by amine molecules.