Two phosphonate-containing bismaleimide (BMI) [(4,4'-bismaleimidophenyl)phosphonate] monomers with different melting temperatures and similar curing temperatures were synthesized by reacting N-hydroxyphenylmaleimide with two kinds of dichloride-terminated phosphonic monomers. The BMI monomers synthesized were identified with H-1-, C-13-, and P-31-nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The phosphonate-containing BMI monomers react with a free-radical initiator to prepare phosphonate-containing BMI polymers and also with various aromatic diamines to prepare a series of polyaspartimides as reactive flame retardants. The polymerization degrees of polyaspartimides depend on the alkalinity and nucleophility of diamines as chain extenders. Differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) were used to study the thermal properties of the phosphonate-containing BMI resins such as the melting temperature, curing temperature, glass transition temperature (T-g), and thermal resistance. All the phosphonate-containing BMI resins, except the BMI polymers, have a T-g in the range of 210-256degreesC and show 5% weight loss temperatures (T-5%) of 329-434 and 310-388degreesC in air and nitrogen atmospheres, respectively. The higher heat resistance of cured BMI resin relative to the BMI polymer is due to its higher crosslinking density. Since the recrosslinking reactions of BMI polymers and polyaspartimides occur more easily in an oxidation environment, their thermal stabilities in air are higher than are those in nitrogen gas. In addition, the thermal decomposition properties of polyaspartimides depend on the structures and compositions of both the diamine segments and the BMI segments.