Research was conducted to study the chemical and physical nature of silicon carbide fiber and graphite yarn surfaces and surface interactions of these fibers with various materials. Various instrumental and analytical procedures were used to characterize fiber surfaces, determine fiber and fiber surface composition and to study the reaction between fiber surfaces and epoxy resin, resin components, coupling agents and model compounds. The purpose of this study is to provide information to guide the development of improved fiber reinforced composites. Silicon carbide fibers analyzed by electron microprobe and X-ray diffraction qualitatively showed the presence of a moderate concentration of oxygen. Silicon carbide fiber surfaces with a specific surface area of 0.016 m2g appear granular by optical microscopic examination but smooth by electron microscopic examination. Surface tension and adhesion tension results showed that oxidation and oxidation-hydration of silicon carbide fibers markedly improved the adhesion between the silicon carbide surfaces and certain coupling agents, and also that the adsorbed film was retained after consecutive treatment with water, toluene, ethyl acetate and boiling water.