Laminated composites consisting of SiC and a thin porous alumina interphase were exposed to air at 500° C to produce a persistent, nearly uniform oxidation product layer. Crack deflection at the interface was then studied using a four-point bend testing procedure and interfacial fracture resistances were found to decrease with increasing oxidation times. Electron microscopy observations of the fractured interface show a complex multi-phase microstructure. These results show that oxidation can produce a sufficiently weak interface in a SiC-porous alumina interphase composite, in contrast to most other SiC composites where interface oxidation produces a strongly bonded interface which inhibits crack deflection.