A series of phosphorus-containing copoly(ester-imide)s was prepared by polycondensation reaction of 2-(6-oxido-6H-dibenz < c,e >< 1,2 > oxaphosphorin-6-yl)-1,4-naphthalene diol, 1, or of various mixtures of 1 and 1,12-dodecanediol, 3, with an aromatic diacid chloride containing preformed imide rings, 2, namely 4-chloroformyl-N-(p-chloroformylphenyl)-phthalimide. The polymers exhibited thermotropic liquid crystalline behavior, as was observed by means of polarized light microscopy, differential scanning calorimetry, and X-ray investigations. The copoly(ester-imide)s showed good thermal stability having the temperature of 5% weight loss above 365-408 A degrees C and char yield at 700 A degrees C in the range 8.8-48%.