A novel, asymmetric diamine 3-(4-amino-phenylithio)-N-aminophthalmide, was prepared from 3-chloro-N-aminophthalmide and 4-aminobenzenethiol. The structure of the diamine was determined via IR and H-1-NMR spectroscopy and elemental analysis. A series of polyimides were synthesized from 3-(4-aminophynylthio)-N-aminophthalmide and aromatic dianhydries by a conventional two-step method in N,N-dimethylacetamide and by a one-step method in phenols. These polymidies showed good solubility in 1-methyl-2-pyrrolidinone, m-cremsol, and p-chlorophenol, except polymide from pyromellitic dianhydride, which was only soluble in p-chlorophenol. The 5% weight loss temperatures of these polyimides ranged from 460 to 498 C in air. Dynamic mechanical thermal analysis indicated that the glass-transition temperatures of the polyimides were in the range 278-395 degrees C. The tensile strengths at break, moduli, and elongations of these polyimides were 146-178 MPa, 1.95-2.58 GPa, and 9.1-13.3%, respectively. Compared with corresponding polyimides from 4,4'-diamidophenyl ether, these polymers showed enhanced solubility and higher glass-transition temperatures. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110:706-711, 2008