[en] A novel series of soluble and curable poly(ether imide) oligomers (PPEI-Phs) containing phthalazinone moiety and terminal phthalonitrile groups were prepared from an excess amount of phthalazinone-based dianhydride and 4,4′-diaminodiphenyl ether, followed by being end-capped with 4-(3-aminophenoxy)phthalonitrile in a two-step, one-pot reaction. The phthalazinone-based PPEI-Phs were cured by a heating treatment procedure up to 350 °C, in the presence of 4,4′-diaminodiphenylsulfone to obtain the cross-linked polymers (cPPEI-Phs). Fourier transform infrared and elemental analysis was utilized to confirm the chemical structures of the resultant oligomers and cross-linked polymers. PPEI-Phs exhibited good solubility in polar aprotic solvents, such as N-methyl pyrrolidone, m-cresol and pyridine, partially soluble in N,N-dimethyl-2-acetamide and chloroform, while the cross-linked cPPEI-Phs were insoluble in all tested solvents, even in sulfuric acid. Furthermore, the gel content of the cPPEI-Phs samples ranged from 96 to 88%, which confirmed the formation of cross-linked network. PPEI-Phs showed high glass transition temperature (T_g) ranged from 225 to 286 °C as determined by differential scanning calorimetry, however, no detectable T_g was observed after thermal curing. cPPEI-Phs exhibited T_d5% in the range of 529–545 °C, which is up to 49 °C higher than the PPEI-Phs, indicating the crosslinking structures of cPPEI-Phs. Moreover, all of cPPEI-Phs displayed high limited oxygen index up to 46.7, which can be attributed to the formation of N-heterocyclic s-triazine or phthalocyanine structures during the curing procedure.