The stability of bismaleimide-o,o’-diallyl bisphenol A (BMI-DABA) blends modified with high-performance amorphous thermoplastic bisphenol A polysulfone (PSF), polyether ketone (PEK-C), and polyether sulfone (PES-C) bearing a phthalidylidene group has been studied by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The extent of stability of thermoplastic components has been compared with the area of the endothermic peak that appears within the glass transition region for thermoplastic component in cured blends aged at temperature below the glass transition temperature (T-g). The stability of thermoplastic can be improved by the formation of semi-interpenetrating polymer networks (semi-IPNs). The stability of thermoplastic with higher T-g is more easily controlled.