Pin-on-disc dry sliding tests were carried out to study the wear mechanisms in a range of metal-matrix composites. 6061-aluminium alloys reinforced with 10 and 20 vol % SiC and Al2O3 particles were used as pin materials, and a mild steel disc was used as a counterface. A transition from mild wear to severe wear was found for the present composites; the wear rate increased by a factor of 10(2). The effects of the ceramic particles on the transition load and wear with varying normal pressure were thoroughly investigated. Three wear mechanisms were identified : abrasion in the running-in period, oxidation during steady wear at low load levels, and adhesion at high loads. A higher particle volume fraction raised the transition load but increased the wear rate in the abrasion and adhesion regimes. Increase of particle size was more effective than increase of volume fraction to prolong the transition from mild wear to adhesive wear. The reasons for different wear mechanisms were determined by analyses of the worn surfaces and wear debris.