PBO [poly(p-phenylenebenzobisoxazole)] fiber has been shown to convert to an ordered carbon fiber without the need for stabilization. This article presents the first detailed analysis of the carbonization and graphitization behavior of this unique material. The carbonization process was modeled as a series of free-radical reactions, and thermogravimetric analysis was used to determine an activation energy of 76 +/- 6 kcal/mol for the thermal initiation of free radicals. The initiation reaction data then were applied to determine the temperature dependence of the termination reaction. Additionally, the development of long-range order in the graphitizing fiber was examined. The spacing between graphene planes was shown to decrease with increasing treatment temperature and soak duration. Carbonized PBO fibers developed more long-range order than carbon fibers produced from other polymers, which may partially explain why these PBO-based fibers display excellent lattice-dependent properties. Finally, an Arrhenius analysis found the activation energy for graphitization to be 120 +/- 17 kcal/mol.