[en] Preparation of high modulus fibers of boron carbide was first reported in 1969 by chemical conversion of a precursor carbon fiber. A method for greatly increasing the strength of the fibers was developed and is described in this work. The mechanism of diffusion of boron into the carbon fiber precursor is considered and data presented to show that carbon also diffuses through the boron carbide coating. Some evidence for a high temperature composition of boron and carbon other than B₄C is discussed. Fabrication of a high temperature composite consisting of B₄C fibers in a carbon matrix and coated with an impervious film of silicon carbide is described. Preliminary data indicates good translation of fiber properties in the composite as well as stability in air at temperatures in excess of 1200⁰C