A recent study has indicated that random copolymers of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate) (PET/PEN) can crystallize over the full composition range under the condition of hot-drawing, and that the crystallites formed are based on random sequences of PET and PEN units. This paper is an extension of this study. Wide-angle fibre X-ray diffraction patterns from PET/PEN random copolymers have been recorded using a charge coupled device based three-circle transmission diffractometer, from which reflections on higher layer lines can be observed quantitatively without distortion. These reflections provide additional indications regarding the tilt of the c-axis away from the fibre axis, the crystallite thickness and the nature of the non-periodic layer crystallites themselves. The long period has also been measured as a function of composition by small-angle X-ray diffraction. Diffraction modelling of the crystallites using the CERIUS package confirms the random nature of polymeric chains in the PET/PEN crystallites and enables the level of register between them to be compared with the ’plane start’ and the non-periodic layer (npl) models. The most appropriate model is an ’npl’, but containing defects in the otherwise perfect sequence register.