The present invention relates to a synergistic flame retardant combination for thermoplastic polymers which comprises as component A a salt of 1-hydroxydihydrophosphole oxides of the formula (Ia) or formula (Ib) and/or 1-hydroxyphospholane oxides of the formula (II) ##STR1## and as component B a nitrogen compound of the formula (III) to (VIII) or a mixture of the compounds designated by the formulae ##STR2## in which the substituents are as defined in the description.