Certain structurally different polyisophthalamides containing pendent phthalimide groups were synthesized from 5-(phthalimide)isophthalic acid and aromatic diamines by the phosphorylation method. Their chemical, physical and thermal properties were determined and compared with those of the corresponding unsubstituted polyisophthalamides. The polymers were characterized by FTi.r., H-1 n.m.r., thermogravimetric analysis, isothermal gravimetric analysis and inherent viscosity measurements. The modified polyisophthalamides showed enhanced solubilities in some solvents, such as 1,4-dioxane, m-cresol and trichloroacetic acid, in comparison to the unmodified analogues. In addition, the incorporation of the pendent phthalimide groups in the polyisophthalamide backbone increased remarkably the thermal stability of the polymer. The modified polyamides were stable up to 317-351-degrees-C in N2 or air and afforded anaerobic char yields of 55-66% at 800-degrees-C. Water absorption and X-ray measurements were also performed.