Carbonized PAN fibers find use as reinforcement in various materials of construction, principally plastic materials.  It is desirable that the fibers have a high tensile strength and for certain purposes that they also have a high electrical resistance. 
Such fibers intended as reinforcement are oxidized by air at a relatively low temperature, typically about 260.degree.  C., and are then subjected to carbonization in an inert atmosphere at an elevated temperature.  The carbonization is typically carried out in an atmosphere of nitrogen and it may be carried out at a relatively low temperature, for example 1200.degree.  to 1500.degree.  C. or at a higher temperature, for example about 2500.degree.  to 3000.degree.  C. The carbonized fibers resulting from the lower temperature carbonization have higher tensile strength than those prepared at the high temperature.