Abstract: A phosphorus-containing cyanate ester which imparts flame retardance to polymers, e.g., thermosetting polymers such as epoxies, with which it is admixed. The resultant polymers are especially useful for electronic, aerospace and automotive applications. The phosphorus-containing cyanate esters have the structure (I): wherein A is a phenyl or naphthyl moiety containing a monocyanato or dicyanato substituent which may be further substituted with one or more of the same or dissimilar straight-chain, branched-chain or cycloalkyl groups having 1 to 12 carbon atoms.