The incorporation of SiOC polymer-derived ceramics into porous carbon materials could provide tailored shapeable, mechanical, electrical, and oxidation-resistant properties for high-temperature applications. Understanding the thermodynamic and kinetic stability of such materials is crucial for their practical application. We report here the dependence of structures and energetics of SiOC and SiOC-modified carbon-bonded carbon fiber composites (CBCFs) on the pyrolysis temperature using spectroscopic methods and high-temperature oxide melt solution calorimetry. The results indicate that a SiOC ceramic pyrolyzed at 1200 degrees C and 1600 degrees C is energetically stable with respect to an isocompositional mixture of cristobalite, silicon carbide, and graphite by 4.9 and 10.3 kJ/mol, respectively, and more energetically stable than that pyrolyzed at 1450 degrees C. Their thermodynamic stability is related to their structural evolution. SiOC-modified CBCFs become energetically less stable with increasing preparation temperature and concomitant increase in excess carbon content.