The invention relates to a new compound having flame-retarding properties of the formula: ##STR2## in which at least one of the groups X is --Y--P(O)(OR).sub. 2 and the two other groups X, independent of each other, are a hydrogen atom, a halogen atom, an alkyl group with at most 6 carbon atoms or --Y--P(O)(OR).sub. 2 or --Y--OH, the symbol Y representing an alkylene group or alkyl- or halogen-sub