The notched and unnotched Izod impact properties of a series of phenolic-glass composites following thermal exposure at 180℃, 300℃, and 800℃ have been investigated. Four phenolic resins; a resol, a novolac, a resol/novolac blend, and a furan-novolac/resol copolymer were used to prepare the composites. The notched and unnotched impact properties of all S-glass composites improved following thermal exposure at 180℃ for times up to 28 days. The best results at 180℃ were obtained for the copolymer-based composite. However, thermal exposure at 300℃ for times greater than 1 day led to significant reduction in the performance of this composite. The best retention of impact properties folowing exposure at 300℃ and 800℃ was found for the composite made with the resol/novolac blend. The results indicate that the impact properties of phenolic composites made with modified resins, that is, a blended resol/novolac or a furan-novolac/resol copolymer resin, improve significantly.