Surface free energy of various carbon fibers was determined by tensiometric method in three different procedures. The dispersive γsd and the nondispersive γsp components were separately determined by contact angle measurements in two-phase system, formamide-n-alkanes. The γsp increased by oxidation and decreased by hydrogen reduction, while the γsd remained almost constant.The γsp value determined from contact angle of water/ethyleneglycol solution tended to become higher. The critical surface free energy was in harmony with the total free energy determined from formamide-n-alkanes system as long as liquids were properly selected. The formation of oxygen-containing functional groups was evidenced by ESCA, and the surface [O]/[C] ratio was related linearly to the surface polarity defined by γsp[γsd+γsp] Oxidation in liquid phase resulted in the formation of both hydroxyls and carboxyls, while only hydroxyls were formed in gas phase. The interfacial shear strength of the fiber-epoxy resin could be closely correlated with thermodynamical work of adhesion calculated on the basis of surface free energy and its component. © 1990 Steinkopff.