The purpose of this study was to determine filler effects on heat stability of vulcanized elastomeric compositions. Carbon-black reinforced compounds, designed to have good heat resistance, were modified by substituting hydrated silica for all or part of the black. Since a simple replacement of silica for black would result in dissimilar original properties, the substituted compounds were further revised to minimize this dissimilarity. This was accomplished through curative correction and/or addition of an organosilane coupling agent. Silica substitution did not provide permanency of properties at elevated temperatures, but in all cases the serviceable life of the compound was extended. Since silica substitution had minimum effect on other properties and reduced the rate of degradation at elevated temperature, this may be considered a reasonable alternative to completely redesigning a proven rubber compound in order to improve its heat resistance.