Mixtures of two crystalline brominated aromatic flame retardants with a surrounding polymer were studied by differential scanning calorimetry, dynamic mechanical analysis (DMA), wide angle X-ray scattering, and solid-state NMR spectroscopy. 1,2-Bis(tetrabromophthalimide)ethane (I) and decabromodiphenyloxide (II) were evaluated in high impact polystyrene (HIPS). Additive I exhibits all the properties of an inert filler. The crystalline structure with respect to the pure material remains unchanged within the polymer. Concerning the HIPS matrix, a slight rise in the melt viscosity and a reduced impact strength as compared to the base resin was observed. In contrast, additive II is miscible with the HIPS matrix and dissolves completely. This leads to an increase of the glass transition temperature of the polybutadiene phase as revealed by solid-state NMR spectroscopy and DMA. The impact resistance of the HIPS matrix is less affected by additive II than by additive I. No interactions were noted with the PS phase at temperatures up to the glass transition.