In the present study, fiber-matrix compatibility in an all-oxide ceramic composite is examined. Reaction-bonding aluminum oxide is used as porous matrix to ensure weak interfaces with fibers. Matrix cracks have been deflected around the interface for sintering temperatures up to 1300°C, due to the effectiveness of porous matrix in enabling damage tolerance. Above 1300°C, densification of the matrix resulted in brittle fracture of the samples, with matrix cracks going through the fibers. Observation of fracture surfaces confirmed the fiber pull-out phenomenon up to processing temperatures of 1300°C. The well-known He and Hutchinson criteria for crack deflection was used to predict debonding behavior at the fiber-matrix interface as a function of matrix porosity. Furthermore, evaluation of the microstructure evolution of Nextel 610 alumina fibers showed a pronounced grain coarsening at sintering temperatures above 1300°C. Changes in crack deflection behavior and fiber microstructure of a composite sample aged for 100 h are also presented.