Polymers crystallized at high temperatures, such as poly(aryl ether ether ketone) (PEEK), poly(ethylene terephthalate) and a thermoplastic polyimide, exhibit well developed lamellar stacks. We have obtained small angle X-ray scattering (SAXS) and wide angle X-ray diffraction data at 290 degrees C and 25 degrees C for PEEK and have also utilized literature SAXS data taken at 25 degrees C for the other materials. To model these systems we assume that the interlamellar ’amorphous’ or non-crystalline species do not contribute to the glass transition and use the SAXS parameters to calculate a total rigid fraction. Using this assumption it is shown that we can qualitatively describe the strength of glass transitions measured directly by thermal analysis. No other rigid ’amorphous’ fractions (RAFs) are needed to model the data under these crystallization conditions. The results suggest that conventionally measured glass transitions originate from the broad amorphous gaps between lamellar stacks in these space-filling spherulitic systems. In situations of very high crystallinities or systems with other crystal morphologies such as defective or high surface area lamellar stack morphologies, the model does not work and the conventional analysis assuming an RAF associated with crystal surfaces is more appropriate.