Abstract: A novel process is provided for the formation of improved carbonized fibrous materials with increased modulus of elasticity and strength as well as decreased diameter. Preferably, the precursor material is acrylonitrile, typically copolymerized with a minor amount of an acrylic monomer such as methyl acrylate. Drawing of the polymer fiber during oxidation is effected at an elevated temperature in the presence of a carboxylic acid (other than formic acid), or its anhydride, within the fiber. It is believed that the acid and/or its anhydride which is formed at the oxidizing temperature serves as a plasticizer and reduces the fiber yield stress and increases fiber plasticity so that the fibers may be drawn by as much as 300% or more in the presence of the acid and/or its anhydride during oxidation, thereby providing the desired improvement in increased modulus of elasticity and tensile strength of carbon fibers formed subsequently by carbonization of the oxidized fibers.