Disclosed is a flame retardant, high impact monovinylidene aromatic polymer composition comprising suitable and sufficient amounts of a rubber modified monovinylidene aromatic polymer, a polyphenylene ether resin, a phosphorous compound, and an optional amount of phenol/aldehye resin as a partial substitute for the polyphenylene ether resin, which composition is rated V-2, V-1, V-0, or 5V in the Underwriter's Laboratories Standard 94 flammability test. More particularly the present invention provides a flame retardant, high impact monovinylidene aromatic polymer composition which can be easily processed comprising: (A) from 40 to 85 percent, based on the total weight of said composition, of comprising a rubber modified monovinylidene aromatic polymer (B) from 10 to 40 weight percent, based on the total weight of said composition, of a polyphenylene ether resin; and (C) from 5 to 20 weight percent, based on the total weight of said composition, of a phosphorous compound. Also disclosed the method of preparing the above-mentioned flame retardant compositions.