In the present study, we elucidate the influence of oxidative heat exposures at 1000 and 1200 A degrees C on an alumina fiber-reinforced polymer-derived ceramic matrix composite containing small residual amounts of carbon. Therefore, we investigated the flexural performance and fracture toughness of on- (0A degrees/90A degrees) and off-axis (45A degrees) reinforced samples. Acoustic emission was used to monitor the internal damage and its progression during loading. At 1000 A degrees C, a moderate reduction of strength and fracture toughness is found while after exposure to 1200 A degrees C a dramatic decrease down to 50 % is observed. For all composites, a reduction of the damaged volume was found after heat treatments indicating a decrease of crack deflection. However, especially at 1000 A degrees C, composites reinforced in 0A degrees/90A degrees direction seemed to be more affected, as no detrimental effect on the mechanical performance was found for the 45A degrees composites. Remarkably, the oxidation-induced silica formation increases the absolute and relative damage thresholds of all composites. A Griffith-like linear relationship between strength and toughness is found. These findings are pivotal for designing and engineering next generation CMCs toward long-term applications.