The glass-transition temperatures and melting behaviors of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PET/PEN) blends were studied. Two blend systems were used for this work, with PET and PEN of different grades. It was found that T-g increases almost linearly with blend composition. Both the Gibbs-DiMarzio equation and the Fox equation fit experimental data very well, indicating copolymer-like behavior of the blend systems. Multiple melting peaks were observed for all blend samples as well as for PET and PEN. The equilibrium melting point was obtained using the Hoffman-Weeks method. The melting points of PET and PEN were depressed as a result of the formation of miscible blends and copolymers, The Flory-Huggins theory was used to study the melting-point depression for the blend system, and the Nishi-Wang equation was used to calculate the interaction parameter (chi (12)). The calculated chi (12) is a small negative number, indicating the formation of thermodynamically stable, miscible blends.