The invention relates to novel and improved halogen-free flame retardant compounds having the structure of Formula (I): wherein: R.sup.1 and R.sup.2 are independently hydrogen, C.sub.1-C.sub.6 alkyl, P(O)(OR.sup.3).sub.2, P(O)OR.sup.3R.sup.4, or P(O)R.sup.3.sub.2, wherein R.sup.3 and R.sup.4 are independently C.sub.1-C.sub.4 alkyl, C.sub.6-C.sub.12 aryl, C.sub.7-C.sub.15 aralkyl or C.sub.7-C.sub.15 alkaryl; or R.sup.1 and R.sup.2 taken together form an unsaturated cyclic ring, which is optionally substituted by an alkyl group; each k is independently an integer from 1 to 2; each X is independently oxygen (O) or sulphur (S); v is 0 or 1; each Y is independently C.sub.1-C.sub.4 alkylene, C.sub.6 arylene, C.sub.7-C.sub.15 aralkylene, C.sub.7-C.sub.15 alkarylene, oxygen (O), nitrogen (NR), wherein R is H or C.sub.1-C.sub.4 alkyl; n is 0, 1 or 2 with the proviso that n is 1 when Y is oxygen (O) or nitrogen (NR); each Z is independently C.sub.1-C.sub.4 alkylene, C.sub.6 arylene, C.sub.7-C.sub.15 aralkylene or C.sub.7-C.sub.15 is alkarylene; m is independently 0, 1 or 2; with the proviso that when Y is oxygen (O) or nitrogen (N), m cannot be 0; each Q is independently C.sub.1-C.sub.4 alkylene; t is an integer from 1 to 2; W is oxygen (O) or sulphur (S). The compounds are particularly suited as flame retardant additives for thermoplastic polyesters.