Flame retardant compositions

The compositions comprise moldable blends formed from at least A) a thermoplastic polyester or thermoplastic polyamide polymer; B) glass fiber reinforcement for the polymer; C) at least one halogen-containing flame retardant; D) an antimony oxide and/or alkali metal antimonate flame retardant synergist; and E) a CTI-enhancing combination of (i) at least one acrylic impact modifier, together with at least one of (ii) and (iii) where (ii) is at least one olefin-based hydrocarbon polymer and (iii) is a zinc borate, a mixture of an alkali metal oxide and a boron oxide, an alkaline earth metal borate, and/or a mixture of an alkaline earth metal oxide and a boron oxide or a zinc borate, or both. Additives and masterbatches composed of certain of these components are also described.