We synthesized a novel phosphorus-containing triamine [9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-yl-tris(4-aminophenyl) methane (dopo-ta)] from the nucleophilic addition of 9,10-dihydro-9-oxa-10-phosphaphenalithrene 10-oxide and pararosaniline chloride, using triethylamine as an acid receiver. We confirmed the structure of dopo-ta by IR, mass, and NMR spectra and elemental analysis. dopo-ta served as a curing agent for diglycidyl ether of bisphenol A (DGEBA) and dicyclopentadiene epoxy (hp7200). Properties such as the glass-transition temperature (T-g), thermal decomposition temperature, flame retardancy, moisture absorption, and dielectric properties of the cured epoxy resins were evaluated. The T-g's of cured DGEBA/dopo-ta and hp7200/dopo-ta were 171 and 190 degrees C, respectively. This high T, phenomenon is rarely seen in the literature after the introduction of a flame-retardant element. The flame retardancy increased with the phosphorus content, and a UL-94 V-0 grade was achieved with a phosphorus content of 1.80 wt % for DGEBA/dopo-ta/diamino diphenylmethane (DDM) systems and 1.46 wt % for hp7200/dopo-ta/DDM systems. The dielectric constants for DGEBA/dopo-ta and hp7200/dopo-ta were 2.91 and 2.82, respectively, implying that the dopo-ta curing systems exhibited low dielectric properties. (c) 2005 Wiley Periodicals, Inc.