The structure of isotropic and highly drawn poly(glycolic acid) (PGA) fibres is investigated using solid-state H-1 NMR spectroscopy. Relaxation times in the rotating frame (T-1rho) using a spin-lock method at 100 degreesC indicate that isotropic PGA can be adequately described by two phases, a rigid crystalline phase and an amorphous mobile phase with relaxation times of 16.6 and 0.9 ms, respectively. A crystallinity of 40% is obtained by this method and agrees well with previous calorimetric studies. In contrast, T-1rho measurements indicate that oriented PGA is 50% crystalline and can be adequately described by a three phase model consisting of. rigid crystalline material with long relaxation time, semi-rigid non-crystalline material with intermediate mobility and relaxation time and highly mobile non-crystalline material with the shortest relaxation time. It was found that the crystalline phase has a T-1rho = 53 and T-1rho = 39 ms when the fibre direction is at 0 and 90degrees relative to the external magnetic field, respectively. This difference in relaxation time is associated with higher spin interactions at 0degrees, reducing the effect on the mobility of the chains. The WAXS orientation averages P-2 =0.99 and P-4 = 0.96 obtained from the azimuthal scans of the (020) and (002) reflections indicate a highly oriented crystalline structure. These results are used to contrast the structural information obtained from measurements and theoretical calculations of the rigid-lattice anisotropy of the second moment (M-2), from which the orientation averages P-2 = 0.96 and P-4 = 0.94 were obtained on the basis of a published crystal structure. The discrepancies found are associated with small differences between the published crystal structure and that required to explain the spin interactions among adjacent molecules. (C) 2004 Elsevier Ltd. All fights reserved.