Two phosphorus-containing heterocyclic flame retardants -9,10-dihydro-9-oxa-10-phosphaphenaiithrene-10-oxide (DOPO) and 2,8-dimethyl-phenoxaphosphin-10-oxide (DPPO) - and their derivatives were characterized and incorporated in the backbone of epoxy novolac to obtain flame-retardant epoxy resins. The structures and spectroscopic data including high-resolution mass spectroscopy of these flame retardants were determined. Flame-retardant epoxy resins with a phosphorus content of up to 2% based on heterocyclic DOPO and DPPO were cured with 4,4-diaminodiphenylmethane (DDM), and their features were examined by UL 94, LOI, and DSC. In this manner, high-performance polymers with glass transition temperatures around 190 degrees C and the UL 94 rating VO were obtained. These polymers were compared with epoxy resins incorporating diphenyl phosphite and diphenyl phosphate, which are nonheterocyclic and do not pass the UL 94 test up to 2%, phosphorus. DPPO has a similar flame retardancy like the commercially available DOPO. Furthermore, to explain the difference in the efficiency of the tested flame retardants, key experiments for the determination of the active species during the flame-retarding process were performed and the PO radical was identified (c) 2007 Wiley Periodicals, Inc.