The research presented in this paper investigates the effect of thermally accelerated aging on the submicrostructure of cellulose and attempts to relate such changes to the well-documented loss of mechanical strength in aged paper. Filter paper and ramie fibers samples were aged in vacuo at 160°C. Small angle X-ray scattering (SAXS) was used to study void structure within the fibers and hydration used as a structural probe. On hydration, the void radius of gyration and area decrease, while the void aspect ratio and overall void fraction increase. After aging, the wet structure more closely resembles the dry, suggesting that water cannot expand the structure to the same extent. It is postulated that increases in local ordering on aging create a structure more resistant to disruption by water. The use of additional techniques, namely Fourier transform IR spectroscopy, wide angle X-ray scattering, scanning electron microscopy (SEM), environmental SEM, and measurement of water retention value, provided additional indirect support for the postulated model. There is no direct evidence for significant crystallinity changes in aged material, suggesting that if structural rearrangements occur, they will be local in nature. There is also no evidence for the formation of covalent crosslinks or new chemical species on aging. Water retention values and wet SAXS results concur, highlighting the importance of water in the cellulose structure and the reduced capacity for water sorption in aged samples. SEM observations show that the failure mechanism in paper changes with age from fibers pull out (i.e., interfiber bond failure) to fiber failure, and wide and zero span tensile tests indicate a weakening of the fibers. These results are consistent with previous reports, and we attribute them primarily to chain scission, although the increased intrafiber bonding may have an influence on the values obtained.