Fiber-coating debonding observed by transmission electron microscopy is suggested as the cause for an similar to60% reduction in strength after one cycle of 1600degreesC heat treatment in a carbon/silicon carbide (C/SiC) composite fabricated by polymer infiltration and pyrolysis with pure allylhydridopolycarbosilane polymer as the matrix precursor. Infiltration with a slurry of SiC powder and the polymer precursor was effective in preserving the coating integrity and composite strength. This study demonstrates that the addition of powder in polymer infiltration and pyrolysis process may impact composite mechanical properties significantly by altering the fiber-matrix interface characteristics.