The increasing use of bioethanol fuel, as a promising carbon-neutral alternative to petroleum-based gasoline, has raised concerns on compatibility issues between bioethanol and the polymer components used in the automotive fuel system. Here, the performance of two commercial glass-fiber reinforced polyamide composites (polyamide 12 (PA12) and an aromatic highly-biobased polyamide (polyphthalamide, PPA)) exposed to a mixture of ethanol and gasoline (E25 fuel), was investigated at high temperature (120 degrees C). The polyamide matrices showed a fast and remarkably high fuel uptake. The sorbed fuel had a strong plasticizing effect on both materials, as revealed by the large decrease in the glass transition temperature and a 60% decrease in stiffness and strength. The PPA, however, sustained degradation better than the PA12, which experienced swelling-induced surface cracking and oxidation-induced embrittlement. The results suggest that care should be taken when using polyamide composites in ethanol-based fuel applications at this high temperature level.