A series of novel addition products (phosphorus content: 0.5, 1, 2, and 3 wt %) were synthesized from the reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and 4,4'-bismaleimidodiphenylmethane (BMI). NMR and IR were used to confirm the structures of the synthetic bismaleimides. Dynamic mechanical analysis scans showed the glass-transition temperatures of these cured BMIs decreased with phosphorus content. Thermal gravimetric analysis heating scans indicated that they had high thermal stability. Limiting oxygen index measurements implied that the flame retardancy was improved by the incorporation of DOPO.