This study examined ethylene-vinyl acetate (EVA)-toughened polystyrene (PS). EVA is well-known to be incompatible with PS; thus, the PS graft to the EVA backbone (EVA-g-PS) was used as a compatibilizer and provided good adhesion at the interface of PS and EVA. In addition, the mechanical properties and impact resistance of the PS matrix were obviously improved by EVA-g-PS and by EVA itself. Meanwhile, differential scanning calorimetry results showed that the grafted PS chain influenced the crystallization of EVA; for example, the melting temperature, the crystallization temperature, and the percentage crystallinity related to EVA were reduced. Moreover, the addition of 10% EVA increased the impact strength by a factor of five but reduced the modulus by the same factor. Additionally, a lower number-average molecular weight EVA delayed phase inversion and resulted in poor mechanical properties. A fracture surface photograph revealed that the major mechanism of EVA-toughened PS was craze and local matrix deformation. (C) 2003 Wiley Periodicals, Inc.