A series of phosphorus-containing nadimide-endcapped resins was prepared by reactingendo-5-norbornene-2,3-dicarboxylic acid anhydride (nadic anhydride), 3,3′,4,4′-benzophenonetetracarboxylic acid dian-hydride, and tris(3-aminophenyl)phosphine oxide in glacial acetic acid/ dimethylformamide. Structural characterization of resins was done by elemental analysis, FTIR, and1H NMR. In DSC traces of these resins, an exothermic transition associated with crosslinking was observed above 280°C. Thermogravimetrie studies revealed a multistep decomposition reaction. Residual weight at 800°C in nitrogen was found to depend on the backbone structure and ranged from 42 to 60%.