Changes in the dynamic response of glass-reinforced phenolic composites following thermal exposure at 180 degreesC for periods of time up to 28 days were monitored using dynamic mechanical analysis. Four phenolic resins were investigated: a resol/ novolac blend, a phenolic-furan novolac/resol graft copolymer, a novolac, and a resol. Reactive blending and copolymerization of phenolic resins are currently being investigated to determine if these techniques will produce phenolic resins land composites) that have improved impact properties and retain the excellent high-temperature properties of resol and novolac phenolic resins. The results indicate that thermal aging at 180 degreesC for 1 day led to a more complete cure of all four phenolic resins as indicated by an increase in the temperature of the maximum of plots of both loss modulus (E") and tan delta versus temperature. The storage modulus (E') of the composites at 40 degreesC varied little following thermal aging at 180 degreesC for 1 day but decreased with increasing exposure time for samples aged 2, 7, and 28 days. Thermal aging led to an increase in E' at higher temperatures and the magnitude of E' at a given temperature decreased with increasing exposure time. The magnitude of E" and tan delta decreased with aging time for all resins, although E" and tan delta were larger for the blend and copolymer composites than for the novolac and resol composites.