Thin films, in the range of tens of micrometers thickness, have been prepared by casting onto glass plates the chloroform or N-methylpyrrolidone solutions of polyimides or poly(imide-amide)s containing silicon and phenylquinoxaline units in the main chain. The polymers have been synthesized by solution polycondensation reaction of aromatic diamines having preformed phenylquinoxaline rings with bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride or with a diacid chloride resulting from the reaction of this dianhydride with p-aminobenzoic acid. The polymers were easily soluble in polar aprotic solvents and showed high thermal stability. The free-standing films exhibited good mechanical properties with tensile strengths in the range of 48-86 MPa, tensile modulus in the range of 1.25-2.22 GPa and elongation at break in the range of 3-37%. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The values of the dielectric constant at 10 kHz were in the range of 2.94-3.08 for polyimides and 3.89-4.49 for poly(imide-amide)s. (c) 2006 Wiley Periodicals, Inc.