Electrical relaxation and conductivity processes in amorphous and semicrystalline poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) homopolymers and certain PET/PEN copolymers have been studied by means of dielectric spectroscopy. Homopolymers and copolymers able to crystallize were subjected to successive thermal runs to investigate the influence of the thermal history upon the morphology and the electrical behavior of the polymeric systems. The morphology of the untreated as well as the heat-treated specimens was determined by means of Differential Scanning Calorimetry (DSC). All samples exhibit beta-relaxation process, due to local motions of the C=O polar side groups, and a-relaxation process associated to the glass/rubber transition. In the PEN spectrum an additional, subglass, mode was recorded, most probably attributed to cooperative motions of the naphthalene groups. Finally, the dynamic nature of the crystallization process is expressed via the over glass transition mode and the temperature dependence of do conductivity recorded in amorphous PET, PEN, and PET/PEN (85/15) (wt/wt) samples. (c) 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3078-3092, 2006.