A fire retardant phosphorous-containing bisphenol compound, bis(3-diethyl phosphono-4-hydroxyphenyl)-sulfide (DPHS), has been inserted in an epoxy network. In particular, a chain extension procedure has been used to incorporate DPHS into a standard bisphenol epoxy resin. Different amine catalysts have been tested to promote the reaction of DPHS with the epoxy oligomer. The cure kinetics of this chain-extended oligomer, using an aliphatic amine (triethylene-tetraamine, TETA) as hardener, has been studied with the aid of differential scanning calorimetry and compared with that of the standard stoichiometric epoxy/TETA formulation. For each formulation the activation energy of the curing reactions has been evaluated by the Kissinger method. The results of thermogravimetric and flammability tests indicate that DPHS promoted the formation of a char, leading to an improved fire resistance. Finally, a comparison of the thermal and mechanical properties of the standard stoichiometric formulation relative to those of the epoxy modified with DPHS, and cured with TETA, showed only marginal changes in the physical and mechanical properties. (C) 2003 Wiley Periodicals, Inc.