Glycidyloxy diphenylphosphine oxide (GDPPO), a side-chain phosphorus-containing monoepoxide, was prepared. The compound, GDPPO, was mixed with 4,4'-diglycidylether of bisphenol A-type epoxy resin, DER 331. The reaction kinetics of the mixture, GDPPO-DER 331, cured with diethyltriamine (DETA), and the thermal decomposition properties of the thermosets were studied. For the GDPPO-DER 331-DETA curing systems, it was found that the activation energy of the curing reaction is 45.0 kJ/mol for the GDPPO-DETA system, and it decreases with the content of GDPPO. According to the method of Ozawa, the average apparent thermal decomposition activation energies of the thermosets with phosphorus-containing monoepoxide are from 138.5 to 178.1 kJ/mol, which are higher than that of the DER 331-DETA system (95.8 kJ/mol). However, the lower crosslinking density and gel fraction of the system with the phosphorus-containing monoepoxide, GDPPO, result in poor protection of the intumescent char during thermal decomposition; thus, the char yield and limiting oxygen index (LOI) of the cured specimens decrease with increasing GDPPO.