This article describes the synthesis, characterization, and thermal properties of nadimides obtained by reacting endo-5-norbornene-2,3-dicarboxylic acid anhydride (nadic anhydride) (NA), 4,4'-oxodiphthalic anhydride (ODA), 1,4,5,8-naphthalene tetra carboxylic dianhydride (NTDA) in glacial acetic acid/DMF. Structural characterization of the resins was done by elemental analysis, IR, H-1-NMR, and C-13-NMR. The DSC scan showed the endothermic transition in the temperature range of 120-270 degrees C. Multistep decomposition was observed in the TG scan of uncured resins in nitrogen atmosphere. Isothermal curing of the resins was done at 250 and 300 degrees C for 1 h in an air atmosphere. These cured resins were stable to (350 +/-30)degrees C and decomposed in a single step above this temperature. This may be due to the retro Diels Alder (RDA) reaction. The char yield of the resins increased significantly on curing. The char yield was highest for P-2N resin and this could be due to the presence of rigid skeleton i.e. naphthalene. (c) 2007 Wiley Periodicals, Inc.