Abstract: An improved process for the thermal stabilization of an acrylic fibrous material or film is provided. The fibrous precursor or film is impregnated with an organic antioxidant by contact with a solution of the same provided at a moderate temperature, dried to remove the solvent, and heated in an oxygen-containing atmosphere at a more highly elevated temperature until a stabilized fibrous material or film is formed. The organic antioxidant advantageously serves to moderate the oxidative portion of the stabilization reaction and enables the cyclization portion thereof to proceed in the absence of substantial polymer chain degradation. The resulting stabilized fibrous material or film is flexible and non-burning, and may be utilized as a fire resistant fiber, fabric or film, or optionally carbonized or carbonized and graphitized to form a carbonaceous fibrous material or film.