Vinylester resin matrix composites were fabricated with 30, 40, 50, and 60% fly ash loading by room temperature casting method. The composites were subjected to thermogravimetric analysis. The 30 and the 60% composites showed a faster degradation at a lower temperature, whereas, the 40 and the 50% composites showed a higher onset temperature. The activation energy was calculated following Broido's equation and was found to be lowered in all the composites compared to the unfilled resin. The residue increased in all the composites proportionately with the increase in the fly ash content. The temperature variation resistance of the unfilled resin, 30 and 60% filled composites were measured and all the samples showed semi conducting nature in 40-60°C temperature range. 29:58-62, 2008. © 2007 Society of Plastics Engineers.