This introduces an organic–inorganic thermosetting hybrid resin system based on unsaturated polyester and polysilazanes. It shows the chemical modification of unsaturated polyester structures by end capping to enable the combination of both components. In general, halogen-free unsaturated polyesters are not fire-retardant and have to be equipped with additives. Fillers and intumescent additives are preponderantly used in today's fire-retardant formulations. In contrast to these fire-retardants, polysilazanes act as ceramizing agents. Polysilazanes are suitable fire-retardants for resin transfer molding due to their low viscosity. Both burning behavior and glass transition temperature (Tg) are investigated as important application properties. In contrast to state-of-the-art fire-retardant formulations polysilazane-based thermosetting hybrid resins burn with high intensity and fast extinction. Therefore, total heat and smoke emission is decreased. The formation of ceramic structures during burning results in high residual mechanical properties and a low mass loss. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40375.