Copolyesters (V-a-V-d and VIa-VId) derived from 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)-1,4-bis(2-hydroxyethoxy) phenylene (II) and bis(2-hydroxyethyl) naphthalate (III) or bis(4-hydroxybutyl) naphthalate (IV) were synthesized and characterized. Copolyesters V-a-V-d, VIa-VId had been characterized by inherent viscosity, solubilities, t.g.a., d.s.c., X-ray diffraction, H-1 n.m.r., P-31 n.m.r., UL-94 and d.m.a. D.s.c. and X-ray diffraction show that the VI series of copolyesters were crystalline polymers and the crystallinity decreased with the increase in II content, whereas the V series copolyesters were amorphous polymers. The inherent viscosity of these copolyesters ranged from 0.53 to 0.68 dl g(-1). The T-g values of the V series copolyesters ranged from 118 to 126 degrees C and those of the VI series copolyesters were in the range 68-77 degrees C; these were higher than the T-g values of polyethylene naphthalate (PEN, T-g: 116 degrees C) and polybutylene naphthalate (PBN, T-g: 67 degrees C). The T-d values (10%) for the V series of copolyesters in nitrogen ranged from 450 to 470 degrees C and those for the VI series copolyesters were in the range 410-415 degrees C; these values were also higher than that of PEN (T-d: 449 degrees C) and PBN (T-d: 410 degrees C). UL-94 V-0 grade could be achieved with a phosphorus content as low as 0.48 wt% for the V series and 0.97 wt% for the VI series copolyesters. T.g.a. kinetics also indicated that the V and VI series copolyesters had higher activation energies of degradation than did PEN and PEN. When these copolyphosphates were treated with fire for 10 s, intumescence is observed on the surface of copolyphosphates which retards further flaming of the polymers. DMA showed that all copolyphosphate esters have good mechanical properties (G＇: 10(8)-10(9) Pa) before glass transition temperatures.