The evolution of microstructure and properties of phenolic resin-based carbon/carbon composites during pyrolysis at different temperatures up to 2500degreesC was investigated. Results of weight loss and open porosity measurements and microstructure observation indicated that a rapid increase of porosity below 600degreesC resulted from the decomposition of phenolic resin matrix, whereas the cracks were developed mainly above 600degreesC. X-ray diffraction analysis showed that carbonization of phenolic resins above 600degreesC resulted in a glassy carbon. As a result, the corresponding C/C composites turned into a brittle-matrix composite, and matrix-dominated brittle fracture behavior with low fiber strength utilization was observed. Weakening of the interfacial bonding after heat treatment above 2000degreesC changed the fracture behavior from a brittle type of failure to a more gracious one. Phenomenon of stress graphitization was observed when the composites were graphitized at 2500degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.