Crosslinked cis-1,4-polybutadiene (P13) rubber is known to undergo crystallization during mechanical deformation. This article presents small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and stress-relaxation studies of the structural development in crosslinked PB samples crystallized under various orientation regimes. The studies are concerned with isothermal crystallization at various temperatures (10 to -50 degrees C) and at constant elongation ratios (1-4.5) and hence with the effects of weak-to-intermediate and high orientations on oriented crystallization versus the behavior of unoriented specimens. The results for weak-to-intermediate orientations indicate a decrease in the long spacing during crystallization, which is accompanied by an almost constant lamellar thickness as well as a significant time lag between the development of crystallinity revealed by WAXS and SAXS intensity measurements. The SAXS integral intensity shows an overshoot; that is, the intensity reaches a maximum after some crystallization time and then decreases. Stress measured during crystallization with constant elongation ratios shows decay, which depends on the isothermal crystallization temperature and elongation ratio. Although the crystallization in the weak-to-intermediate orientations is interpreted on the basis of nucleation growth, the crystallization under high orientations is best interpreted on the basis of spinodal crystallization. The most pronounced feature of structural formation during crystallization under high orientations is an almost constant long period from nearly the beginning of the crystallization, even in a timescale in which other physical quantities such as the WAXS crystallinity, SAXS integral intensity, and stress still change with time, suggesting spinodal-like crystallization, orientation-induced instability of molten network chains and their ordering, which involves spinodal-elecomposition-like segregation of noncrystallizable parts (crosslinked points) and crystallizable parts (linear chains between the crosslinks) of network chains, and subsequent crystallization of crystallizable chains and pinning of the ordering process induced by crystallization. To the best of our knowledge, this work reports for the first time spinodal-like crystallization on oriented crosslinked polymer melts. (C0 2007 Wiley Periodicals, Inc.