Carbon/carbon (C/C) composite materials are widely used in high-temperature fields because of their outstanding thermal-physical properties. But many difficulties have been experienced in the joining of C/C composites because of their brittleness and the harsh application environments. Use of high-temperature adhesives has been shown to be one of the most convenient and promising methods for the joining of materials employed at high temperatures. In this work, C/C composites were bonded by an organic resin adhesive using phenol-formaldehyde (PF) resin as matrix and boron carbide as modification additive. The shear bond strength of C/C composite joints, after being treated at 1200 degrees C, were tested at room temperature, 1000 degrees C, 1400 degrees C and 1800 degrees C. Results show that the above organic resin adhesive possesses satisfactory bond properties. The bond strength of C/C composite joints tested at room temperature was as high as 13.2 MPa. Because of the modification effect of boron carbide, the thermal stability and integrity of the residue derived from the carbonization of PF resin are improved considerably, which is responsible for the achievement of satisfactory bond strength. As to the C/C composite joints tested at high temperatures, the bond strength is obviously lower than that of C/C composite joints achieved at room temperature due to the melting of boron oxides which were the products of boron carbide's modification reactions. The propagation of failure is much easier in boron oxides glass phase and is an important factor resulting in the decrease of bond strength at high temperatures. With the increase of test temperature, the amount of the boron oxides produced decreased because of the carbon thermal reduction reaction and its volatilization. As a result, the bond strength of C/C joints increased from 2.3 to 5.2 MPa with the increase of test temperature from 1000 to 1800 degrees C. (C) Koninklijke Brill NV, Leiden, 2009