A series of phosphorus-containing copolyesters was prepared by polycondensation of 2-(6-oxido-6H-dibenz < c,e >< 1,2 > oxaphosphorin-6-yl)-1,4-naphthalene diol, 1, or of an equimolecular amount of 1 and different bisphenols 2, such as 4,4'-isopropylidenediphenol, 4,4'-(hexafluoroisopropylidene)diphenol and 2,7-dihydroxynaphthalene, with an aromatic diacid chloride containing two preformed ester groups 3, namely, terephthaloyl-bis-(4-oxybenzoyl-chloride). The copolyesters exhibited good thermal stability having the temperature of 5% weight loss above 380 degrees C and char yield at 700 degrees C in the range of 20-31.2%. The glass transition temperature was in the range of 136-147 degrees C. The polymers exhibited thermotropic liquid crystalline behavior, as was observed with polarized optical microscopy, differential scanning calorimetry, and X-ray experiments. POLYM. ENG. SCI., 50:1028-1035, 2010. (C) 2010 Society of Plastics Engineers