The significance of particle cavitation in rubber-toughened polymers remains controversial. While some researchers believe void formation promotes shear yielding in the polymer matrix, others consider it as a secondary energy consuming process. The research described here was undertaken to further elucidate the role of particle cavitation in toughening through comparative examination of epoxies modified by conventional rubber modifiers and hollow plastic particles. The results of this study illustrate that rubber particles with different cavitation resistance and pre-existing microvoids toughen the present epoxy matrix in the same manner. Therefore, we conclude that the cavitation resistance of the rubbery phase does not directly contribute to toughness, but instead simply allows the matrix to deform by shear. An additional mechanism of microcracking was observed when 40-mu m hollow plastic particles were employed. Despite the similar behaviour in fracture toughness testing, rubber particles and microvoids differ considerably in how they affect the compressive yield strength of the blend. The results of this study suggest the possible importance of inter-particle distance in toughening of epoxies. This concept will be examined in part 2 of this study.