Carbon composites containing novolac/furfuryl alcohol resins with hexamethylene tetramine (HMTA) as a crosslinker and carbon black or mesophase pitch as an additive, were heated to 1000 degrees C under Argon atmosphere. Carbonization chemistry was studied including the nitrogen-containing structures in the final carbons. The volatiles released during the heating were furfuryl, phenol-benzene species, methane, ethane and other small molecular species such as moisture, CO2, CO and ammonia. When carbon black was used as an additive in the composite, a considerable amount of furfuryl and phenolic species were "trapped" on the surface of carbon black particles, resulting in a higher carbon yield. Certain carbonization reactions also occurred at lower temperatures, and the dimensional shrinkage was reduced as compared to the resin-only system. On the other hand, the mesophase pitch additive formed homogeneous morphologies with the resin binder, participated in the carbonization process of the resin binder and formed amorphous carbons with porous structures and dimensional expansion. Manipulating the composition and variety of the additives could produce carbon composites with designed performance. (C) 2011 Elsevier B.V. All rights reserved.