Two kinds of phosphonate-containing bismaleimide (BMI) monomers, phenyl-(4,4'-bismaleimidophenyl) phosphonate and ethyl-(4,4'-bismaleimido-phenyl) phosphonate, were synthesized and added through blending to two epoxy systems for the study of their applications as reactive flame retardants. The thermal behaviors of the BMI monomers in both kinds of epoxy systems, bisphenol and phenol-novolac, were similar. An increase in the BMI contents increased the storage modulus and glass-transition temperature but slightly reduced the mechanical strength of the epoxy blends. The pyrolysis models of both BMI blends in the two epoxy systems were quite alike. Although the initial pyrolysis temperatures of all the blending systems gradually decreased as the phosphorous content increased, the flame retardancy of all the phosphonate-containing epoxy systems was promoted significantly by increasing contents of BMI. (C) 2004 Wiley Periodicals, Inc.