Phase transitional behavior has been investigated for a series of ethylene (E) and tetrafluoroethylene (TFE) two-component copolymers on the basis of the temperature dependent measurement of X-ray fiber diagrams taken for the uniaxially-oriented samples. The usage of such uniaxially-oriented samples has allowed us to clarify the structural changes more definitely than before. So far, the crystalline transition from the low-temperature phase to the high-temperature phase had been assumed to occur continuously, but the detailed analysis of the X-ray reflections at higher scattering angles revealed for the first time that the transition is of the thermodynamically-discontinuous first-order type between the monoclinic and the pseudohexagonal phases. This phase transition temperature shifted toward lower temperature side as the TFE content became higher. The analysis of the X-ray reflection profile allowed us to imagine that low-temperature shift of the transition point is considered to occur due to the easier thermal motion of the planar-zigzag chains around the chain axis because of looser packing of chains containing higher population of bulky CF2CF2 groups. From these experimental data a phase diagram has been built up to show the phase transition behaviors of a series of E/TFE copolymers in a systematic manner. The characteristic features of the structural disordering including trans-gauche conformational exchange have been also discussed on the basis of the data of polarized infrared and Raman spectra combined with the X-ray diffuse scatterings. (C) 2008 Elsevier Ltd. All rights reserved.