Dielectric properties of Halar, a predominantly alternating 1 : 1 copolymer of ethylene (E) and chlorotrifluoroethylene (CTFE), have been obtained as a function of temperature (−100 to +175°C), frequency (102–106 Hz), and thermal history. The dielectric loss index (ε″), related to heat dissipation in electrical applications, shows no major change between 25 and 175°C. However, a significant increase in (ε″) between +25°C and −100°C is not considered detrimental for the applications. Activation energies (δE) for the three major relaxations (α-, β-, and γ-) in both mechanical and dielectric experiments are similar, thus suggesting a similar phase origin of the molecular relaxations in the two techniques. In dielectric analysis, the lowest temperature γ-relaxation is the strongest while it is the weakest in mechanical technique. It is proposed that, in dielectric experiments, only the CTFE groups participate and due to restricted mobility at low temperatures (i.e., γ-relaxation), the dissipation factor or relaxation strength is maximum.