This study analyses the effects of heat treatments in oxidative atmosphere on the mechanical and microstructural properties of a fiber-reinforced weak interface composite (UMOX (TM)) which is composed of a mullite-SiOC matrix and Nextel (TM) 610 fibers with fugitive coatings. Composites of different porosity grades, depending on the polymer infiltration and pyrolysis cycle, are exposed to 1000 and 1200 degrees C for 50 h. The exposure provokes the formation of silica, which leads to matrix densification and the formation of silica bridges at the fiber-matrix interface, resulting in an increased interfacial bonding strength. Consequently, the fracture toughness and the flexural strength are significantly reduced. The study confirms that SiOC-based materials are suitable for an application at high temperatures in oxygen-rich atmospheres up to 1000 degrees C. It is, however, important to consider the microstructural changes and thereby induced decrease of the overall mechanical performance during a high-temperature use.