The aim of the present work was to investigate the high temperature reaction between Si- C-O fibres and Cl2, and to correlate the carbon layer growth kinetics with their initial composition and structure. Large disparities have been observed between the various materials reaction rates. If a large amount of mixed silicon atom environments increases the reactivity of Si-O-C materials with Cl2, this sole feature does not explain all the experimental results. Traces of heteroelements (Ti, Zr, Al…) or percolated free carbon both appear to bear on the reaction rate. For a better understanding of the fibres reaction behaviour, this study was extended to other model materials (oxycarbide glasses, oxygen-free Si-C fibres, SiC).