Six impact modifiers were prepared using the polyester adducts formed by reaction of maleic anhydride and poly(ethylene glycols) (PEG’s) or poly(propylene glycols) (PPG’s) of differing molecular weights. A number of toughened resins were prepared by blending different proportions of modifiers with commercially available general-purpose unsaturated polyester resin. The impact modifiers containing PEG-based modifiers were compatible with the resin, and those based on PPG were incompatible. The tensile strengths and modulii of the toughened resin compositions containg 10% loading of the incompatible modifiers were comparable to those of the base resin. The toughened resin compositions containing incompatible modifiers exhibited better tensile strengths and moduli. The resin toughness showed up to 5-fold improvement with the use of the impact modifiers. Resin modified with compatible modifiers exhibited higher toughness. Dynamic mechanical properties confirmed compatibility behavior of modifiers.