Al-Cu matrix composites with a high volume fraction of alumina particles (41-62%) prepared by gas-pressure infiltration are characterized in tension and chevron-notch fracture testing before and after heat-treatment. Their mechanical behaviour is shown to depend markedly on the matrix structure and flow stress, and also on the nature and size of the reinforcement particles. Al-Cu matrix composites free of coarse Al2Cu matrix intermetallics and reinforced with 60 vol% high-strength polygonal alumina particles exhibit strength/toughness combinations that are in the same range as unreinforced high-strength aluminium alloys: the strength of the composites can be increased without decreasing their toughness. The results are interpreted on the basis of current cohesive zone models for crack propagation by microcavitation in elastic-plastic materials. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.