The excellent high-temperature mechanical properties and other desirable characteristics of polybenzimidazole (PBI) polymer systems make these systems attractive candidates for development as ablative heat-shield materials. This paper describes the formulation of several new low-density polybenzimidazole composites. The proposed structure of the basic linear PBI prepolymer and of several highly cross-linked PBI polymers are presented. The cross-linked PBI's were obtained either thermally (by postcuring to a high temperature) or chemically (by the use of either preoxidized polyfunctional amines or triphenyl trimeasate as a comonomer in the polymerization).