The miscibility behavior and morphology of a series of poly(vinyl butyralco-vinyl alcohol) (PVBA) copolymers containing 29, 52, 76, and 88 mol % of vinyl alcohol units with poly(ethylene terephthalate-co-ethylene naphthalate) (PETN) was investigated by DSC and SEM. Blends of the PETN with PVBA were prepared by coprecipitation from a chloroform/o-chlorophenol (20/80 wt %) mixture solvent. It was found that PVBAs with different vinyl alcohol content will form an immiscible phase with the amorphous PETN in the solution-cast films. Also, PETN and PVBA with 29 mol % vinyl alcohol (PVBA-29) are not miscible within the whole composition range. The glass-transition temperatures of the blends were higher than those of the two-component polymers. The values of the tensile properties of the blend films were also better than those of the original copolymer films.