Structural changes during deformation in solution- and gel-spun polyacrylonitrile (PAN) fibers prepared with multi- and single-wall carbon nanotubes, and vapor-grown carbon nano-fibers were investigated using synchrotron X-ray diffraction data. Deformation of carbon nanotubes (CNTs) contributes to the increased modulus. CNTs, in addition to their mere presence as reinforcement, were found to alter the response of the PAN matrix to stress and thus enhance the performance of the composite. CNTs facilitate the orientation of the PAN crystals during deformation, and increase the load transferred to PAN crystals as evidenced by their increased lateral and axial strains at 75 degrees C. The monotonical decrease in PAN interchain spacing with the fiber strain was accompanied by a reversible helix to zigzag conformational change as well as by changes in the axial repeats of the two conformations. These changes were much larger in gel-spun fibers than in solution-spun fibers, indicating more effective load transfer in gel-spun fibers. (C) 2008 Elsevier Ltd. All rights reserved.