High-strength corrosion-resistant austenitic steel 03Kh14N11K5M2YuT with deformation-unstable austenite is studied. Regular features of the evolution of the substructure of the steel are investigated. The anomalously high ductility of the steel is shown to be explainable by the joint action of uniform slip, microtwinning, and martensitic transformations responsible for formation of subfine martensite crystals.