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 (T-m1’, T-m1, and T-m2 at increasing values) strongly depend upon thermal treatments. The higher-temperature melting peak T-m2 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-m1 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 T-m2. The peak at T-m1 can be due to crystals that are able to recrystallize and perfect during thermal treatments. Lastly, the peak at the lowest-temperature T-m1’, 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 ethylene-chlorotrifluoroethylene (ECTFE) alternating copolymer.