9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is an organophosphorus compound that is used to produce fire retardants.

The reactivity of the P-H center allows for the conjugation of DOPO to many monomers.

Such monomers are incorporated into polymers, conferring fire retardant properties.

[3] The compound is prepared from 2-phenylphenol, which condenses with one equivalent of phosphorus trichloride.

Heating that species in the presence of aluminium trichloride results in cyclization.