Measurements of wide-angle diffraction during high speed spinning of poly(ethylene terephthalate) (PET) have been performed using synchrotron radiation. The experimental set-up has been improved so that it became possible to determine the degree of crystallinity, crystallite orientation and fiber diameter along the spin line. For take-up speeds of 3600 m/min and higher, the crystal reflections appeared at a distance of 40 cm from the spinneret, just after the necking region. The crystallization speed increases proportional to increasing take-up speed. For the take-up speeds investigated, this results in a constant crystallinity profile when measured as a function of the distance from the spinneret. The final fibers exhibited an extraordinarily high degree of crystallinity, which has been attributed to the high molecular weight of the polymer. Below 3500 m/min, no crystallization and no necking could be detected up to a distance of 90 cm from the spinneret. The results showed that necking and crystallization were closely coupled. No orientation of the amorphous matrix prior to crystallization could be detected by means of X-ray diffraction. Moreover, the orientation of the crystallites was constant along the spin line. (C) 2000 Elsevier Science Ltd. All rights reserved.