In this study, we synthesized a phosphorus-containing triply functionalized reactive modifier, DOPO-tris(azetidine-2,4-dione), and a phosphorus-free doubly functionalized reactive modifier, bis(azetidine-2,4-dione), and embedded them into epoxy resin systems. We characterized these synthesized reactive modifiers using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, elemental analysis, and mass spectrometry. During the thermosetting processes, we reacted the epoxy curing agents 4,4-diaminodiphenylmethane and tris(4-aminophenyl)amine with the multiply hydrogen-bonding reactive modifiers and epoxy monomers. The introduction of the DOPO segment, strongly hydrogen bonding malonamide linkages, and hard aromatic groups into the backbones of the synthesized reactive modifiers resulted in epoxy networks exhibiting tailorable crosslinking densities, flexibilities, glass transition temperatures, thermal decomposition temperatures, and flame retardancies. Furthermore, dynamic mechanical analyses indicated that intermolecular hydrogen bonding of these reactive modifiers enhanced the thermal and physical properties of their epoxy resins through the formation of unique pseudocrosslinked polymer networks. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2411-2420, 2011