Polystyrene and its blends with styrene/alpha-methylstyrene and styrene/dicyclopentadiene copolymers were modified by addition of Ph3P or Bu3PO flame retardants and optionally the montmorillonite filler via premix masterbatches in Brabender extrusion plastometer at 190-210 degrees C for 15 min to reduce flammability of the products. The 0 index of the modified polystyrene was increased from 17,2% to 22,3%. Its thermal stability was also increased. The modification resulted however in a substantial decrease in the tensile strength, stress and elongation at break and modulus of elasticity of the products.