Several aromatic polyisoimides were prepared, and their third-order nonlinear susceptibilities, chi((3))(-3 omega, omega, omega, omega), were measured by optical third-harmonic generation. Obtained chi((3)) values are in the order of 10(-12) e.s.u. at a fundamental wavelength of 1907 nm. The largest value, chi((3)) = 3.4 x 10(-12) e.s.u., was Obtained for the polyisoimide prepared from pyromellitic dianhydride (PMDA) and p-phenylenediamine (PDA). Thermal stability of the polyisoimide prepared from PMDA and PDA was tested. The contents of isoimide structure decreases due to thermal imidization above 200 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.