(relative to using unsized fibers) with < wt.% of the sizing in the composite. The thermoplastic sizing dissolved in the thermoset at the cure temperature, bur microphase separated into similar to 90 nm diameter inclusions during matrix curing. A gradient in chemical composition and morphology resulted, which moved outward from the fiber into the thermoset matrix. It is hypothesized that the unusually good fatigue resistance of composites from these: materials can be at least partially attributed to this interphase gradient.