Low temperature plasma treatment experiments were conducted on AS4 and AU4 type fibers in the presence of air, argon and nitrogen to study their effects on the resulting fiber-matrix adhesion in the composites fabricated with the fibers. Optimum conditions for the introduction of maximum surface oxygen and nitrogen functional groups on AS4 and AU4 fiber surfaces were derived. X-ray photelectron spectroscopy was performed on the fiber surfaces to determine the effect of plasma treatment on the amount and types of surface functional groups introduced. Scanning electron microscopy was performed on the fiber surfaces and the amount of surface etching and pitting was correlated to the plasma treatment conditions. Transverse tensile specimens were fabricated from treated fibers representing a few plasma treatment conditions, and their transverse tensile strengths were measured experimentally. An attempt was made to determine the effects of factors such as amount of chemical groups on the fiber surfaces, mechanical interlocking, molecular level entanglement and physical interactions to explain the data.