A series of novel semi-2-interpenetrating polymeric networks (semi-2-IPNs) were prepared through blending in solution using two different polyimides, biscitraconamic acid as a precursor of biscitraconimide (MBMI) with various proportions of polyetherimide (PEI) to achieve optimum properties. Biscitraconamic acid was prepared by reacting citraconic anhydride (CA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and bis(3-aminopropyl)phenyl phosphine (BAPPP) and it was characterized by differential scanning calorimetry (DSC), FTIR, and H-1-NMR spectroscopy. Both biscitraconamic acid and PEI were blended in N,N-dimethylacetamide (DMAc) solution, casted and thermally cured up to 300 degrees C to give semi-2-IPNs. The MBMI/PEI semi-IPN systems were characterized by UV-Vis spectroscopy, FTIR spectroscopy and thermal techniques. The phase morphology, isothermal aging, and water uptake of semi-IPN systems have also been studied. The morphological studies on phase distribution were investigated by scanning electron microscopy (SEM). Thermal performance of MBMI/PEI semi-IPN systems were evaluated by DSC and thermo gravimetric analysis (TGA). All the compositions of semi-IPN polyimide system were stable up to 400 degrees C and their thermal stability increased with increase in the content of PEI. Isothermal aging studies done at 300 degrees C for various time periods showed good thermo-oxidative stability. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2790-2799, 2011