Pyrolysis of a Yajima-type polycarbosilane (PCS) has been performed under an atmosphere of hydrogen on both bulk samples and model monofilaments up to 1000 degrees C, in order to reduce the free carbon content of the resulting ceramics. The organic/inorganic transition occurs with in the 400-800 degrees C temperature range, with mainly an evolution of CH4. At 1000 degrees C, it yields an hydrogenated amorphous ceramic with a C/Si atomic ratio and a free carbon content significantly lower than for its counterpart obtained under inert atmosphere (namely, 1.18 and 9 at% versus 1.72 and 27 at%). Hydrogen is thought to favour the release of the pendent methyl groups of the PCS via demethanation radical reactions. Continuous model filaments were produced via the melt spinning of the PCS, electron beam curing, pyrolysis under hydrogen up to 1000 degrees C, and a final heat treatment under argon up to 1600 degrees C. The ceramic fibres exhibit a C/Si atomic ratio of 1.10, a free carbon content of approximate to 8 at%, a Young’s modulus of 260-300 GPa and a tensile failure stress of 2100 MPa. Their thermal stability is limited to 1400 degrees C due to some oxygen contamination during the process.