In an attempt to minimize the acetaldehyde formation at the processing temperatures (280–300°C) and the outer–inner transesterification reactions in the poly (ethylene terephthalate) (PET)–poly(ethylene naphthalate) (PEN) melt-mixed blends, the hydroxyl chain ends of PET were capped using benzoyl chloride. The thermal characterization of the melt-mixed PET–PEN blends at 300°C, as well as that of the corresponding homopolymers, was performed. Degradations were carried out under dynamic heating and isothermal conditions in both flowing nitrogen and static air atmosphere. The initial decomposition temperatures (Ti) were determined to draw useful information about the overall thermal stability of the studied compounds. Also, the glass transition temperature (Tg) was determined by finding data, indicating that the end-capped copolymers showed a higher degradation stability compared to the unmodified PET and, when blended with PEN, seemed to be efficient in slowing the kinetic of transesterification leading to, for a finite time, the formation of block copolymers, as determined by 1H-NMR analysis. This is strong and direct evidence that the end-capping of the [BOND]OH chain ends influences the mechanism and the kinetic of transesterification. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers