Functionalized silica sols were obtained by the hydrolytic condensation of (γ-methacryloxypropyl)trimethoxysilane (MPMS), (γ-glycidyloxypropyl)trimethoxysilane (GPMS) and tetraethoxysilane (TEOS). Three different sols were obtained: MPS (derived from MPMS and TEOS), GPS-MPS (derived from GPMS, MPMS and TEOS), and GPSD (derived from GPMS, TEOS and diglycidyl ether of bisphenol A, DGEBA). These silica sols were mixed with a phenolic resin (PR). Ethylenediamine was used as a hardener for epoxy-functionalized sols and benzoyl peroxide was used as an initiator of the free-radical polymerization of methacrylate-functionalized silica sols. Glass fiber-reinforced composites were obtained from the neat PR and MPS-PR, GPS-MPS-PR and GPSD-PR. The resulting composites were evaluated as ablation resistant materials in an acetylene-oxygen flame. A large increase in the ablation resistance was observed when the PR was modified by the functionalized silica sols. The ablation resistance of the composites decreased as follows: GPSD-PR > MPS-PR > GPS-MPS-PR > PR.