Polysulfonamide (PSA) short fiber/ethylene-propylene-diene elastomer (EPDM) composites were prepared as high-performance thermal insulators. The pyrolysis products and thermal stabilities of the PSA fiber and aramid fiber were analyzed. The ablation world thermal properties of the PSA/EPDM composites were compared with those of the aramid/EPDM composites. The degradation peak temperature of the PSA fiber was 100 degrees C higher than that of the aramid fiber. With the fiber content increasing the ablation rates of both composites decreased up to 10 phr and subsequently increased whereas the thermal conductivities of both composite increased linearly. hi comparison with the aramid/EPDM composites, the ablation rate and thermal conductivity of the PSA/EPDM composites were lower, whereas the thermal degradation temperature and limiting oxygen index of the PSA/EPDM composites were higher. Dynamic mechanical thermal testing and morphology observations revealed that strong interfacial and morphology observations revealed that strong interfacial bonding between the fiber and the matrix was developed in the PSA/EPDM composites, and it was beneficial for improving the ablation properties of the composites (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 283-289, 2009.