Silicone-based additives have been used as fire retardants for thermoplastics, presenting the advantages of improving processing and impact resistance of the polymers. In this work we used three different silicone-based additives as modifiers of a thermoset based on a vinyl ester resin. The additives are fine powders made up of about 50 wt % ultra high molecular weight polydimethylsiloxane and 50 wt % silica. The differences between them are the functional groups inserted on the additives and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % of styrene. For curing the mixture a conventional catalyst and initiator were used and the reaction was carried out in two ways, differing in the curing temperature, the post curing temperature, and the time, and in the addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. The samples were characterized by thermogravimetric analyses and swelling experiments. The fire retardances of the samples were evaluated by the determination of the flash-ignition, self-ignition, and pyrol-ysis temperatures (ASTM D1919-91a), and of the oxygen index (ASTM D-2863-91). The results obtained showed that the silicone-based additives and the methods used in the preparation of the modified resin influence the flash-ignition, self-ignition, and pyrolysis temperatures, but not the oxygen index. Samples cured by different methods present different network characteristics, which influence their thermal decomposition. The volatile species produced by thermal decomposition may be a combination of inert and active species. The network structure may influence only the inert fraction of the volatiles, not the combustibles. These volatile inert species (smoke-black, water vapor, carbon dioxide, etc.) probably dilute the combustibles in the solid and in the gaseous phase, increasing the flash-ignition temperature of the samples. (c) 2005 Wiley Periodicals, Inc.993644649