Accelerated UV degradation conditions were imposed on different epoxy resin composites reinforced with carbon fibre, 3D parabeam glass, and E-glass. X-ray micro computed tomography (XμCT) was used to evaluate the changes in the composites’ inner structure as a result of the treatment. Structural damage factors such as density changes, reinforcement filler damage, homogeneity, cracks and microcracks were examined. XμCT 3D images, 2D reconstructed slices and void calculations showed cracks in all composites with different shapes and volume in response to the UV degradation conditions. 3D-glass reinforced epoxy composites showed the least amount of damage in comparison with other composites. E-glass suffered the most damage upon UV degradation. Most of the structural damage in both 3D-glass and E-glass composites occurred in the resin matrix in the near-surface region, while internal damage was present in the form of delamination in the carbon fibre composite.