Silicone-based additives have been used as fire retardants for thermoplastics and present the advantages of improving the processing and impact resistance of the polymers. In this study, we used three different silicone-based additives as modifiers of a vinyl ester resin. The additives were fine powders made up of about 50 wt %, polydimethylsiloxane and 50 wt % silica. The differences among them were the functional groups inserted in the polymer chains and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % styrene. To cure the mixture, a conventional catalyst and initiator were used, and the reaction was carried out in three ways, which differed in the curing temperature, post-curing temperature, time, and addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. Dynamic mechanical analysis showed that the phase behavior of the resulting composites depended strongly on the curing conditions. The flexural modulus of composites containing 5 wt % additive was lower than that for the cured resin. The impact resistance of the composites also depended on the curing conditions but not on the composition or size of the particle of the additive. The fracture morphologies of specimens subjected to impact resistance tests were different for samples cured in the presence or in the absence of DMA, which suggested that it influenced the mechanism of network formation. (c) 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99:2279-2287, 200699522792287