A series of copolymerized polyimide (PI) thin films with low dielectric constants were prepared with different molars ratio of bis[3,5-dimethyl-4-(4aminophenoxy)phenyl]methane and 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene as diamines and 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) as a dianhydride. Some films possessed good dielectric properties with an ultralow dielectric constant of 2.3 at 1 MHz. The structures and properties of the thin films were measured with Fourier transform infrared and NMR spectroscopy, thermogravimetric analysis, and dynamic mechanical analysis. The PI films exhibited glass-transition temperatures in the range 223-243 degrees C and possessed initial thermal decomposition temperatures reaching up to 475-486 degrees C in air and 464-477 degrees C in nitrogen. All of the PI films exhibited excellent solubility in organic solvents. The mechanical properties of these films were also examined. (C) 2008 Wiley Periodicals, Inc.