A lightweight ablator formulation has been developed which offers superior thermal performance compared to current state of the art ablator formulations. The lightweight ablator formulations described herein typically include at least one endothermically decomposing (energy absorbing) material with a fluxing agent resulting in significantly reduced backface temperature response and a more stable surface. According to one implementation the ablator composition comprises about 30 to about 70 percent by weight of a base silicone resin, about 25 to about 67 percent by weight of a low-density filler, about 3 to about 7 percent by weight of a curing agent and greater than 0 and up to about 10 percent by weight of a boron-containing compound.