Phosphino oxide group containing compounds useful as flame retardant additives, having the general formula (I): ##STR1## wherein: X and X.sub.1 represent a hydrogen atom or, when taken jointly, they represent an NR.sub.1 moiety in which R.sub.1 stands for a hydrogen atom; a linear or branched aliphatic C.sub.1 -C.sub.5 moiety; a linear or branched C.sub.2 -C.sub.6 acyl moiety; a phenyl moiety;R represents a cyclic or polycyclic C.sub.4 -C.sub.9 moiety containing two carboxy groups, possibly esterified with univalent alcohols or glycols, or transformed into anhydrides, or two hydroxy groups.