A series of perylene-containing random copolyimides with different ratios of diaminododecyl perylene tetracarboxylic dianhydride (PPI-12) and 4,4′-(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) have been studied. Copolymerization improves the solubility and the copolyimide with a low percentage (less than 20% of total dianhydrides) of perylene becomes highly soluble in conventional solvents, such as, CHCl₃, CH₂Cl₂, or THF etc. The positions of the absorption maxima in the UV–visible spectra do not change with 6FDA concentration, but the relative intensities do. Although the homopolymer PPI-12 does not show a distinct glass transition, its T_g was derived by extrapolation of the DSC results with the copolymers. The copolymers are semi-crystalline up to a concentration of 60% (mol) 6FDA. It is of interest that the spacings of X-ray reflections also do not change with the addition of 6FDA. This is rationalized on the basis that the conformation of the 6FDA segment does not cause any significant difference in the preferred conformational shapes of the homopolymer of PPI-12. Due to this “conformational isomorphism” 6FDA is not a significant chain “kinker” in this case.