New phosphorus-containing aromatic diesteramines, 1,4-bis(4-aminobenzoyloxy)-2-(6-oxido-6H-dibenz[c,e][1,2] oxaphosphorin-6-yl)naphthalene (p-3) and 1,4-bis(3-aminobenzoyloxy)-2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)naphthalene (m-3), were synthesized by the reaction of 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)-1,4-naphthalenediol with 4-nitrobenzoyl chloride and S-nitrobenzoyl chloride, respectively, followed by catalytic reduction. Two series of phosphorus-containing aromatic poly(ester-amide-imide)s with inherent viscosities of 0.94-2.00 and 0.41-0.56 dL/g were prepared via low-temperature solution polycondensation from p-3 and m-3, respectively, with three imide ring-preformed diacid chlorides. All the poly(esteramide-imide)s were amorphous and readily soluble in many organic solvents such as N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP). Transparent, tough, and flexible films of these polymers were cast from DnlAc or NMP solutions. Their casting films possessed a tensile strength range of 118-181 MPa, an elongation to break of 5-11%, and an initial modulus range of 2.41-4.46 GPa. They had useful levels of thermal stability associated with relatively high glass-transition temperatures (264-286 degreesC) and 10% weight-loss temperatures in excess of 450 degreesC in nitrogen or air. The limiting oxygen indices of these polymers were 41-46.