Understanding the importance of synergistic effects that occur in combined radiation (dose rate R) plus thermal aging (temperature T) environments has been of interest for many years. Although suggested approaches for achieving this objective are contained in two recent international publications, the validity of these approaches is questioned. A new approach is described based on chemical kinetic principles, and applied to elongation data for two elastomeric materials: a chloroprene and a chlorosulfonated polyethylene. At low temperatures and high dose rates, the chemistry from radiation-initiation totally dominates the degradation, leading to the rate constant kR in the radiation limit. This rate constant is shown to be independent of temperature as long as the dose rate at that temperature is high enough to reach radiation-limit conditions. For combined environment experiments at R + T, this result, added to the rate constant kT appropriate to the thermal limit (obtained from thermal-only exposures), leads to the combined environment rate constant expected in the absence of synergism. Comparing this result to the experimental combined environment rate constant kR+T leads to quantitative estimates of synergism across (R-T) space. Important synergistic effects are found for both materials.