9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, also known as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) Chemicalbook, is an important new reactive phosphorus-containing flame retardant. Flame retardants synthesized from DOPO and its derivatives have the characteristics of high efficiency, halogen-free, smoke-free, non-toxic, non-migrating, and long-lasting flame retardancy.