Block copolymers of bisphenol A isophthalate and sebacate were prepared by a two-step interfacial polycondensation. The copolymers had sebacate contents of 1 : 5 and 1 : 3 molar with block lengths (DP) of 4,9, and 19. Thermal analysis of the copolymers by DSC showed the Tm of PBI. The crystallizability of PBI chain segments in the block copolymers was confirmed by X-ray diffraction. The Tm of the copolymers was primarily influenced by the DP of the chain segments rather than the sebacate content. The Tg of the copolymers was mainly affected by the sebacate content rather than the DP of chain segments. The independent Tg of sebacate in the copolymers was not detected by DSC, i.e., the two polymer chains were compatible without phase separation. Phase separation was only observed in the rheometric spectrum of a 1 : 1 blend of the two homopolymers. The sebacate, being compatible with the isophthalate, had a plasticizing effect upon the polyarylate, which resulted in an improvement in toughness of the copolymers.