AbstractThe unit cell of an alternating copolymer of ethylene and tetrafluoroethylene was determined by x‐ray diffraction. In spite of uncertainties due to irregularities in the chain structure and a low level of crystallinity, a reasonable unit cell and structure was derived which gives a calculated crystalline density of 1.9 g/cm3. The unit cell is believed to be either orthorhombic or monoclinic with the following parameters:a= 9.6 Å,b= 9.25 Å,c= 5.0 Å, (γ = 96°). The molecular conformation is that of the extended zigzag, and the molecular packing appears to be orthorhombic, each molecule having four nearest neighbors with the CH2groups of one chain adjacent to the CF2groups of th