Two methods for preparing precursors of Si/C/B-based thermostructural materials have been developed. The first consists of a thermal treatment of a mixture of polydimethylsilane (PDMS), (Me2Si)(n), and triethylamine-borane adduct, Et3N : BH3 at atmospheric pressure. The second involves two steps : (i) transformation of PDMS into a polymer displaying Si-CH2-Si and Si-Si linkages in its backbone, and (ii) heating this product at atmospheric pressure, in the presence of Et3N:BH3. The ceramic material obtained from the second approach contains 2.2 at% boron and 1.5 at% oxygen.