the condition of the carbon fiber surface is of importance in determining the mechanical properties of a carbon - carbon (c - c) composite. the presence of functional groups on the fiber surface and roughness of the surface control to a great extent the tensile strength, fracture toughness and interlaminar shear strength. a compromise has to be made between a weak interfacial bond which improves fracture toughness by the debonding and fiber pull-out, but results in a low tensile strength and a strong interfacial bonding which leads to catastrophic failure. this paper demonstrates the use of surface treatments and de-treatments to optimize the mechanical properties of c - c composites. oxygen plasma and nitric acid treatments were used to introduce the surface functional groups and to alter the surface morphology. an argon plasma de-treatment was used to reduce the surface functionality. a correlation between the carbon surface modifications and the mechanical properties is discussed with emphasis on the fundamental understanding of the fiber-matrix interactions. (edited author abstract) 32 refs