The crystalline structure of polyamide-12 (PA12) was studied by solid-state C-13 nuclear magnetic resonance (NMR) as well as by synchrotron wide- and small-angle X-ray scattering (WAXS and SAXS). Isotropic and oriented PA12 showed different NMR spectra ascribed to gamma- and gamma'-crystalline modifications, respectively. On the basis of the position of the first diffraction peak, the isotropic gamma-form and the oriented gamma'-form were shown to be with hexagonal crystalline lattice at room temperature. When heated, the two PA12 polymorphs demonstrated different behaviors. Above 140 degrees C, the isotropic gamma-PA12 partially transformed into alpha-modification. No such transition was observed with the oriented gamma'-PA12 phase even after annealing at temperatures close to melting. A gamma'-gamma transition was observed here only after isotropization by melting point. Various structural parameters were extracted from the WAXS and SAXS patterns and analyzed as a function of temperature and orientation: the degree of crystallinity, the d-spacings, the Bragg's long spacings, the average thicknesses of the crystalline (l(c)) and amorphous (l(a)) phases, and the linear crystallinity x(cl) within the lamellar stacks. (c) 2005 Wiley Periodicals, Inc.