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• A novel composite ablator based on silicone resin and B 4 C ceramics is shown. • Methylpolysiloxane ablation rates significantly reduced by formed B 2 O 3 on surface. • Methylpolysiloxane pyrolyses to SiOC and SiO 2. • The presence of B 4 C promotes β-SiC crystallisation during pyrolysis and ablation. Before siloxane resin matrix composites could be successfully used as ablative materials, their mechanism of high-temperature degradation must be investigated in depth. In this study, degradation mechanism, ablation resistance and degradation kinetics of both methylpolysiloxane and the methylpolysiloxane - B 4 C (10 wt%) composite were investigated.. The presence of B 4 C resulted in a higher final sample mass in thermogravimetric tests in synthetic air up to 1500C (85% vs 81%), while during ablation tests it reduced linear and mass ablation rates by 45% and 54%, respectively. The XRD and SEM/EDS studies provided an estimate of the phase contributions formed as a result of exposure to corrosive conditions. The effect of the presence of powders on the amount of SiO 2 glassy phase, β-SiC crystalline phase and free carbon precipitates formed is apparent. [ABSTRACT FROM AUTHOR]