Background: Spacecraft operating in Very Low Earth Orbit (VLEO, 200–400 km) are subjected to high atomic oxygen (AO) fluxes (up to 10⁸ atoms/cm³ at ∼7.5 km/s), which strongly erode polymeric and composite materials. This study evaluates the AO resistance of additively manufactured carbon-PEEK (C-PEEK) and a TiO₂-coated variant. Material durability was assessed using the ENEA ground-based AO beam facility following ASTM E2089, reproducing representative VLEO conditions. Methods: The coupled thermal and oxidative response was investigated through thermal cycling under controlled oxygen flux. Surface and sub-surface modifications were analyzed using optical microscopy, SEM, profilometry, and roughness mapping. Chemical alterations were examined via XPS and XAS to identify oxidation pathways. Results: Thermal cycling caused an initial outgassing phase that stabilized after the first heating steps. Uncoated C-PEEK exhibited moderate mass loss, decreased apparent density, and localized surface erosion with fiber exposure. Conversely, TiO₂-coated sample displayed only minor morphological changes and negligible roughness variation. Conclusions: After correction for volatile release, C-PEEK erosion rates were consistent with reference data, supporting its suitability for VLEO use. The TiO₂ coating markedly improved AO resistance, indicating its effectiveness as a protective strategy for spacecraft materials in oxidative LEO environments. © 2026 Elsevier B.V.

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