The effects of diffusion across filament-matrix interfaces and of weak interfacial bonds on the axial fatigue resistance of boron-6061 aluminum composites were determined. Interfacial microstructures and composite fatigue resistance were compared with those previously determined for current, state-of-the-art boron-6061 aluminum composites. It was concluded that (a) the fracture behavior of the silicon carbide (SiC) coating on coated boron filaments preferentially degrades the low-cycle fatigue resistance of composites, (b) weak interfacial bonds extend the fatigue life of boron-6061 aluminum composites, (c) increased diffusion penetration across boron-6061 aluminum interfaces shortens composite fatigue life by a detectable but minimal amount, and (d) the fatigue resistance of current, state-of-the-art boron-6061 aluminum composites can be improved substantially