How chemical structure changes affect the curing reactions and thermal properties of 2,2'-bis(4-cyanatophenyl) propane (cyanate ester) cured rigid-rod epoxy resins has been studied for biphenol and tetramethyl biphenol epoxies. The formation of an oxazolidinone ring is the dominant curing reaction for biphenol epoxy, whereas the formation of a cyanurate ring is the dominant curing reaction for tetramethyl biphenol epoxy, due to the bulky methyl substitutions on the biphenol chain. The crosslinkable cyanurate structure has a higher thermal stability than that of the linear oxazolidinone structure, giving the eyanate ester cured tetramethyl biphenol epoxy resin (molar equivalent) a higher glass transition temperature (260 vs. 218degreesC) and thermal decomposition temperature (366 vs. 312degreesC) than that of cyanate ester cured biphenol epoxy resin.