This study focuses broadly on synthesis and characterization of porous mullite/alumina mixtures for use as matrices in oxide fiber composites. Specifically, an assessment is made of the utility of a precursor-derived alumina (PDA) in controlling both the modulus and the toughness of mullite-rich particle mixtures. Property changes are probed through models of the mechanical behavior of bonded particle aggregates. Consideration of the conditions needed to cause crack deflection at a fiber-matrix interface yields an upper allowable limit on the concentration of PDA to ensure damage tolerance in a fiber composite. The predicted critical concentration lies in the range of about 7-9%, depending on the mullite/alumina ratio in the particle slurry and the subsequent aging treatment. Values slightly below this limit should yield composites that exhibit a desirable balance between fiber- and matrix-dominated properties.