The material used to form carbon fibers is not a homopolymer of acrylonitrile, as the latter is rather difficult to dissolve and form into shaped articles; instead, the material is a copolymer containing in excess of 85 wt% acrylonitrile. The formation of PAN-type fibers suitable for conversion into high strength or high modulus carbon fibers is a complex, multivariate process. This chapter focuses on conversion of rayon into carbon fibers, one of the expensive processes in carbon fiber manufacture is the stabilization step. On the other hand, the fiber must be carbonized rapidly to keep some nitrogen in the structure in order to increase molecular flexibility, leading to high strength carbon fibers. Research and development will continue to reduce the cost of manufacture of PAN-based carbon fibers, primarily through reduction of the stabilization costs. The acrylic precursor is soluble in hot aqueous sulfuric acid.