Wear experiments have shown that when the applied normal stress exceeds a critical value, a transition occurs from the regime of mild wear to that of adhesive wear. As a result, the wear rate of particle-reinforced aluminium composites increases by a hundred-fold. Based on dislocation and delamination theories, a criterion is proposed for determining the critical transition load, and a quantitative model is developed for adhesive wear in a wear system of a steel disc sliding against aluminium matrix composite pins. Experimental results of four kinds of metal-matrix composites confirm the validity of the criterion and the model.