The formation of lamellar structures in polyacrylonitrile (PAN) fibers was investigated in the temperature range from 25 to 500 degrees C using synchrotron small-angle X-ray scattering. It was found that, in addition to a first lamellar mesophase showing up at about 200-300 degrees C, a second lamellar mesophase appears in an inert atmosphere at temperatures higher than about 350 degrees C. The former was attributed to the cyclization, the latter to the aromatization reaction of PAN. The periods of the first (alpha) and the second (beta) lamellar phase are about 13 and 5 nm, respectively. Both the alpha and the beta phase show preferred orientation. The average angle between the fiber axis and the normals of the lamellae is about 15 degrees. Homopolymer and copolymer fibers differ considerably in the kinetics of developing the alpha phase.