Poly(ethylene- co -chlorotrifluoroethylene) with a 1∶1 molar ratio of ethylene and chlorotrifluoroethylene in composition, PECTFE, is a statistically alternating copolymer widely used in cable coating industry. PECTFE has been commonly characterized as a semicrystalline polymer with crystals melting at 238 ℃. Differential scanning calorimetry, thermal mechanical analysis and dynamic mechanical analysis indicated that PECTFE undergoes a reversible solid-solid transition during cooling and heating in the temperature range between 100 ℃ and 200 ℃. Based on detailed structural analysis via wide angle X-ray diffraction and electron diffraction techniques, we proposed that PECTFE possesses a hexagonal phase above 200 ℃( a =0 60 nm, c =0 49 nm) and an orthorhombic phase( a =0 99 nm, b =0 57 nm, c =0 49 nm) at room temperature. Polyethylene (ethylene-co-chlorotrifluoroethylene) with a 1: 1 molar ratio of ethylene and chlorotrifluoroethylene in composition, PECTFE, is a statically-dispersed copolymer widely used in cable coating industry. PECTFE has been characterized as a semicrystalline polymer with a melting temperature of 238 Differential scanning calorimetry, thermal mechanical analysis and dynamic mechanical analysis indicated that PECTFE undergoes a reversible solid-solid transition during cooling and heating in the temperature range between 100 ° C and 200 ° C. Based on detailed structural analysis via wide angle X-ray diffraction and electron diffraction techniques, we proposed that PECTFE possesses a hexagonal phase above 200 ° C (a = 0 60 nm, c = 0 49 nm) and an orthorhombic phase (a = 099 nm, b = 0 57 nm, c = 0 49 nm) at room temperature.