Applications of commercial engineering polyesters like poly(butylene terephthalate) (PBT) in transportation, aviation and construction require, among other parameters, a high level of flame retardancy. Flame retardancy is often achieved by adding halogen-containing flame retardants (FR). Recent regulations [1] restrict a number of most efficient compounds. Therefore, the goal of the project was to develop polymeric FRs with polyester backbone for halogen-free flame retardancy with high FR efficiency, preventing blooming of the FR to the surface and not affecting the mechanical property level of the base polyester PBT. In presentation we will outline the influence of the chemical structure of different phosphorus-containing polyesters on their thermal degradation, pyrolysis and burning behavior. Polyesters with 9,10-dihydro-9-oxa-10-phospha-phenantrene-10-oxide (DOPO) DOPO substituents generally showed the best FR potential with respect to high char formation, reduction in total heat release, fire growth rate etc. combined with the occurrence of intumescence [2,3]. After analysis of the neat phosphorus polyesters, they were employed as FR additives in PBT. Their influence on processing, burning behavior and resulting mechanical properties will be discussed [4]. In a next step, phosphorus-containing PBT nanocomposites with either layered silicates or multiwalled carbon nanotubes were prepared and the additional influence of nanoobjects was evaluated. References [1] WEEE (2012/19/EU); RoHS (2011/65/EU). [2] Fischer, O.; Pospiech, D.; Korwitz, A.; Sahre, K.; Häußler, L.; Friedel, P.; Fischer, D.; Harnisch, C.; Bykov, Y.; Döring, M.: Synthesis and properties of phosphorus-polyesters with systematically altered phosphorus environment, Polym. Degrad. Stab. (2011), 96, 2198-2208. [3] Brehme, S.; Köppl, T.; Schartel, B.; Fischer, O.; Altstädt, V.; Pospiech, D.; Döring, M. Macromol. Chem. Phys. 2012, 213, 22, 2386-2397. [4] Köppl, T.; Brehme, S.; Pospiech, D.; Fischer, O.; A