High temperature wide-angle X-ray diffraction (HTWAXD) has elucidated the differences in the structural changes in undrawn and crimped sulfonated poly(ethylene terephthalate) (SPET) fibres during heating from 230 degrees C through melting. Both the heat-induced crystals and the drawing-induced crystals in the undrawn and crimped SPET fibres are triclinic. The HTWAXD data suggest that different morphologies are responsible for the distinct melting behaviour between the undrawn and drawn SPET fibres. It is thought that spherulites and bundled crystals are induced by heating and drawing in the undrawn and drawn SPET fibres, respectively. Upon heating, the undrawn fibres crystallize into spherulitic crystals (T-m = 257 degrees C) whose structure do not change at temperatures approaching melting. The drawing-induced bundle crystals in the crimped fibres melt at a lower melting temperature of 249 degrees C. The bundled crystals are oriented to the fibre axis, but highly stressed, less perfect, and smaller in size, therefore melt at lower temperatures. With increasing temperatures, the partially melted paracrystalline structure transforms into a pseudohexagonal crystalline structure before reorganizing into spherulites which melt subsequently.