Structural changes during deformation in solution- and gel-spun polyacrylonitrile (PAN) fibers with multi- and single-wall carbon nanotubes (CNTs), and vapor-grown carbon nanofibers were investigated using synchrotron X-ray scattering. Previously published wide-angle X-ray scattering (WAXS) results showed that CNTs deform under load, alter the response of the PAN matrix to stress, and thus enhance the performance of the composite. In this article, we find that the elongated scattering entities that give rise to the small-angle X-ray scattering (SAXS) in solution-spun fibers are the diffuse matrix-void interfaces that follow the Porod's law, and in gel-spun fibers these are similar to fractals. The observed smaller fraction of voids in the gel-spun fibers accounts for the significant increase in the strength of this fiber. The degree of orientation of the surfaces of the voids is in complete agreement with those of the crystalline domains observed in WAXS, and increases reversibly upon stretching in the same way as those of the crystalline domains indicating that the voids are integral parts of the polymer matrix and are surrounded by the crystalline domains in the fibrils. The solution-spun composite fibers have a larger fraction of the smaller (<10 nm) voids than the corresponding control PAN fibers. Furthermore, the size distribution of the voids during elongation changes greatly in the solution spun PAN fiber, but not so in its composites. The scattered intensity, and therefore the volume fraction of the voids, decreases considerably above the glass transition temperature (T-g) of polymer. Implications of these observations on the interactions between the nanotubes and the polymer are discussed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2394-2409, 2009