The influence of constitutional defects (those resulting from variations of the comonomer molar ratio from 50/50 and those due to the introduction of termonomer units) and thermal history on the polymorphic behaviour of ethylene-tetrafluoroethylene (ETFE) copolymer is investigated. At low temperatures, while well developed orthorhombic structures are obtained for copolymers with a C2F4 content in the range sufficient to favour formation of the so-called pseudo-hexagonal mesophase. At high temperatures, the degree of crystallinity as well as the correlation length of the hexagonal domains remain essentially unaltered by an increase of the C2F4 molar content from 50 to 80%, while both are strongly reduced by low concentrations (1-3 mol%) of the termonomer units (substantially excluded from the crystalline phase). This is expected to be a determinant factor for the improved ultimate properties typical of commercial terpolymers of ETFE.