SiC preforms were infiltrated with polycarbosiloxane preceramic polymer to create low-cost SiC—SixOyCz matrix composites for possible high temperature application. The effect of specimen annealing conditions on the steam and air oxidation resistance was analyzed. Air exposures from 900–1500°C showed stability of the SixOyCz matrix phase with moderate oxidation resistance. After 900°C exposures, oxidation of the composite is primarily limited to the matrix phase, and only at 1200 and 1500°C was SiC consumption visualized. Steam cycle exposures at 1200°C resulted in rapid linear oxidation of the composites with induced specimen swelling from volume expansion associated with internal oxidation. Specimens annealed in argon were found to be the most oxidation resistant due to increased matrix crystallinity and carbon retention. The results of this work were used to develop clear strategies for improving the high-temperature properties of SiC—SixOyCz composites.