Poly((L)-lactide) (PLLA) was melt blended with a set of polyethylenes (PE) in an effort to increase the impact strength of the PLLA. As compatibilizers, we prepared a series of molecularly distinct polylactide-polyethylene block copolymers. The influence of the copolymer structure on the matrix/dispersed phase interfacial adhesion was correlated with the mechanical properties of the PLLA composites. For the lowest modulus PE dispersed phase, the block copolymer that gave the strongest interfacial adhesion was necessary to achieve the most significant improvement in toughening. Whereas with the stiffest PE, the block copolymer that gave the weakest interfacial adhesion resulted in the greatest improvement in impact strength. For the intermediate stiffness PE, an intermediate degree of adhesion was necessary to obtain the largest increase in the impact properties. The impact properties of the composites were also found to be highly dependent on the dispersed phase properties. (C) 2004 Elsevier Ltd. All rights reserved.