The instant disclosure relates to phosphorus-containing compounds that can be used to form flame retardant phosphorus-containing resins, and also can serve as a hardener for a flame retardant epoxy resin compositions. In particular, the phosphorus-containing compounds are modified with acyloxy groups (—O—(C═O)—R), as shown below. Incorporation of the acyloxy groups results in resins that are water resistant and exhibit improved dielectric properties.