Short silica fibre-reinforced simultaneous interpenetrating polymer network (IPN) composites consisting of polymethylsilsesquioxane (PMSQ) and phenol-formaldehyde resin were prepared through thermal cross-linking. Such IPN composites combine the advantageous ablative features of individual systems and excellent thermal insulation aspects. IPNs were prepared with varying composition between PMSQ and PF, at constant fibre content (60% by weight). The influence of IPN composition on the ablation and out-gassing characteristics was explored in detail. Arc-jet evaluation brought out the superior thermal shock resistance of IPN composites that were richer in PMSQ content. Further, it was inferred that the mechanism of thermal protection shifted to ablative heat shielding with increase in PF content. Microstructural characterization of the arc plasma eroded specimens was carried out using FTIR and Raman spectroscopy to understand the chemical mechanism of erosion process. Higher PMSQ content facilitated the formation of SiOC glass, whereas higher PF content resulted in well-defined char layer comprised of predominantly pyrolytic carbon. Higher PMSQ content also favoured lower out-gassing properties. The study brought out the superior thermal protection capability of the IPN with respect to aerospace applications.