The double bonds and aromaticity that impart high thermal stability in fibers often increase the absorption of LTV radiation, resulting in greater susceptibility to photodegradation. This study compares the resistance of three high-performance fibers (Nomex aramid, Kynol novoloid, and PBI) to accelerated light exposure. X-ray photoelectron spectroscopy (XPS) was used to evaluate the photochemical changes on the surfaces of the fibers. On the basis of the C 1 s core-level spectra and oxygen/carbon atomic ratios, the Nomex aramid fibers were photo-oxidized more rapidly than Kynol novoloid and PBI fibers. Similarly, Nomex aramid fibers exhibited more rapid strength losses than the other fibers. We propose that tautomerization inhibited photodegradation in the Kynol novoloid and PBI fibers. XPS core-level and valence-band spectra interpreted by ab initio calculations are shown to be valuable probes of photo-oxidation in fibers.