In an attempt to obtain flame-retardant epoxy systems with high thermal properties, novel phosphorus-containing naphthyl epoxies were synthesized through the addition reaction of diethyl phosphite and two naphthyl epoxy monomers, diglycidyl ether of 2,2'-hydroxyl-1,1'-binaphthalene and 1,1'-diglycidyldinaphthylmethane. The addition reaction was monitored with Fourier transform infrared spectroscopy and solubility experiments, and the reaction conditions were studied. The spectral results showed that oxirane in phosphorus-modified epoxies could undergo an isomerization process in which carbonyl groups were generated during the reaction at temperatures higher than 140 degrees C. The structure and epoxy equivalent weight of the modified naphthyl epoxies were determined by means of H-1-NMR and P-31-NMR. The thermal curing reactivity and activation energy of the phosphorus-containing naphthyl epoxies were investigated with differential scanning calorimetry with dicyanodiamide and diaminodiphenylsulfone as the curing agents, and they were compared with those of an epoxy modified with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 541-546, 2009