Water soluble diamine amic acids (DAAs) were synthesized by reacting aliphatic diamines with pyromellitic dianhydride. Poly(amide-amic acid)s (PAAs) were prepared by interfacial polycondensation of DAAs in aqueous sodium hydroxide solution with isophthaloyl chloride in dichloromethane. Poly(amide-imide)s (PAIs) containing alternating (amide-amide)-(imide-imide) sequences were obtained by thermal cycloimidization of the PAA films at 175 degrees C for 4 h in a forced air woven. The PAIs were readily soluble in polar aprotic solvents such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, and N-methyl-2-pyrrolidone. The inherent viscosities of the polymers are in the range of 0.97-1.7 dL/g. The polymers were characterized by IR, H-1 nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA). Thin film composite membranes containing PAA ultrathin barrier layer were prepared by in situ interfacial polycondensation of DAA in water with trimesoyl chloride or isophthaloyl chloride in hexane on the surface of a porous polysulfone membrane. The membranes were characterized for water permeability and for the separation of NaCl and Na2SO4. (C) 2000 John Wiley & Sons, Inc.