The orientation and crystal size of a series of melt-spun high acrylonitrile AMLON(R) and solution-spun commercial acrylic fibers are investigated by optical and X-ray diffraction analysis. The effect of different processing steps is examined. The image intensity of a birefringent fiber under cross polarizers is fitted accurately, assuming that the birefringence across the entire fiber cross-section is uniform. The molecular orientational order parameter of the fiber, as measured by X-ray diffraction, correlates linearly with its birefringence. The orientation of the melt-spun fibers is as good or better than that of commercial solution-spun fibers, and it increases dramatically in the melt-spun fibers when they are post drawn or spun from higher molecular weight resins. Orientation is affected only slightly by annealing processes, which in turn increase the crystal size in the fiber. (C) 2000 Elsevier Science Ltd. All rights reserved.