Impact resistant polymer blends of crystallizable polyester, e.g. PET, and styrene copolymer graft rubber composition, e.g. ABS, and an impact modifying amount of a mixture of a fatty acid salt, e.g. sodium stearate, and an acid copolymer, e.g. SMA, are useful as engineering thermoplastics, e.g. for injection molding of articles requiring high impact resistance.