Blends of a novel phosphorus containing PEN copolyester (PENP2) and poly(butylene naphthalate) (PBN) have been studied before and after transesterification. According to DSC results, the physical blend exhibits complete miscibility when the content of PEN is less than 40% or higher than 90% by weight and exhibits partial miscibility outside these ranges. Transesterified blends were obtained by holding the physical blend at 260-280 degrees C. The transesterified systems show a single T-g and exhibit a marked melting point depression. P-31 and H-1 NMR and wide-angle X-ray diffraction were also performed in this article. (C) 2000