A series of aromatic copolyethers containing 1,3,4-oxadiazole rings and phthalide groups was prepared by nucleophilic substitution polymerization technique of phenolphthalein, 1, or of an equimolecular amount of 1 and different bisphenols 2, such as: 4,4'-isopropylidenediphenol, 4,4'-(hexafluoroisopropylidene)diphenol, 4,4'-(1,4-phenylene-diisopropylidene)bisphenol, 4,4'-cyclohexylidene-bisphenol and 2,7-dihydroxynaphthalene, with 2,5-bis(p-fluorophenyl)-1,3,4-oxadiazole, 3. The polymers were easily soluble in polar solvents such as N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and chloroform and can be cast from solutions into thin flexible films. They showed high thermal stability, with decomposition temperature being above 400 degrees C. The polymers exhibited a glass transition temperature in the range of 220 - 271 degrees C, with reasonable interval between glass transition and decomposition temperature. 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 and 20 degrees C were in the range of 2.98 - 3.15. (c) 2007 Elsevier Ltd. All rights reserved.