Poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) homopolymers were synthesized by the two-step melt polycondensation process of ethylene glycol (EG) with dimethyl terephthalate (DMT) and/or dimethyl isophthalate (DMI), respectively. Nine copolymers of the above three monomers were also synthesized by varying the mole percent of DMI with respect to DMT in the initial monomer feed. The thermal behavior was investigated over the entire range of copolymer composition by differential scanning calorimetry (DSC). The glass transition (T-g), cold crystallization (T-cc), melting (T-m), and crystallization (T-c) temperatures have been determined. Also, the gradually increasing proportion of ethyleno-isophthalate units in the virgin PET drastically differentiated the tensile mechanical properties, which were determined, and the results are discussed.