Polyimides are attractive for high temperature applications as the matrix resins, due to their excellent thermo-oxidative stability and mechanical properties. As composite materials for aerospace applications, they possess certain unique characteristics such as toughness, resistance to temperature and solvents as well as high tensile strength and modulus. A series of novel PMR polyimides based on substituted benzidines are examined and compared to the state-of-the-art PMR-15. The assigned glass transition temperatures and the mechanism for the thermal decomposition of four specific PMR polyimides are obtained, using thermal mechanical analysis and TGA/FTIR/MS techniques, Their thermal decomposition steps are proposed, identified and also compared to each other.