Poly(1,4-butylene terephthalate) (PBT) is an important engineering thermoplastic polymer which is very often reinforced by glass fibers (GF) to improve its mechanical properties. Applications of GF-PBT could be limited due to its high flammability and flame retardants (FRs) must be incorporated into the polymer. In this work, the selected FRs are aluminum diethylphosphinate (OP 1240) and a combination of melamine cyanurate and of aluminum diethylphosphinate (OP 1200). Synergists are investigated in formulations of GF-PBT containing OP1200 or OP1240 according to different fire scenarios (limiting oxygen index (LOI), UL-94 and mass loss calorimetry). It is found that polyhedral silsesquioxanes (POSS)-based molecules are synergists of interest combined with OP1200 and OP1240. The combination of the layered potassium acetate modified kaolin with OP1200 also appears to be beneficial. Introduction Engineering thermoplastic polymers such as polyamides, polycarbonates, semicrystalline aromatic polyesters and their blends are widely used in electrical and electronic appliances [1]. Poly(1,4-butylene terephthalate) (PBT) is one of them and it is a very important engineering thermoplastic polymer since it combines several desirable properties such as high strength and rigidity, low moisture absorption, excellent electrical properties and chemical resistance, rapid molding cycles and reproducible mold shrinkage [2]. It is very often reinforced by glass fibers to make various composites with high performance. Glass fibers are incorporated into PBT in order to improve properties such as stiffness, strength, creep and to increase the dimensional stability of the polymer [3]. The amount (20-50 wt.-% typically) and length of fibers as well as their orientation in PBT are governing factors that enhance