The melting behavior of the alternating copolymer of ethylene and tetrafluoroethylene (ETFE) was investigated by differential scanning calorimetry (DSC). A complex melting pattern is observed, strongly influenced by the thermal history. Three melting processes can be identified, whose heats of fusion and peak temperatures (Tml,, Tml, and Tm2 a t increasing values) strongly depend upon thermal treatments. The higher-temperature melting peak Tm2 is not affected by the crystallization conditions (i.e., cooling rate and annealing); therefore, it can be attributed to more perfect crystals present in the original sample. The peak at T,,,, increases when the cooling rate is decreased and upon prolonged and/or higher-temperature annealing, and it fastly merges to the higher-temperature peak at Tm2. The peak a t T,, can be due to crystals that are able to recrystallize and perfect during thermal treatments. Lastly, the peak a t the lowest-temperature Tmt,, produced only by annealing and strongly developing on increasing annealing time and temperature, is the so-called annealing peak that can be attributed to much poorer crystallites grown among the larger ones. The melting behavior of ETFE is compared with that of the ethylenechlorotrifluoroethylene (ECTFE) alternating copolymer.