The resistances of annealed pure metals to solid particle erosion and two-body abrasion increases linearly with increasing hardness and thus are similar. However, alloyed metals show a contrasting behaviour. For example, a substantial increase in the hardness of steels achieved by quenching and tempering does not alter the erosion resistance. The abrasion resistance, however, does increase, although not as dramatically as in the case of pure metals. The objective of this paper is to understand the reasons behind such a contrasting erosion and abrasion behaviour with respect to hardness. A detailed analysis of the nature of plastic deformation during erosion and abrasion in conjunction with the estimates of the average strain rates at which the plastic deformation occurs within the plastic zone, leads to the conclusion that deformation occurs under fully adiabatic conditions only during erosion and not under abrasion conditions. This conclusion, when incorporated in the localization models for erosion and abrasion proposed earlier by the present author, results in theoretical expressions for erosion and abrasion resistance which have different dependences on hardness. Thus the differential effects of hardness on erosion and abrasion are explained. © 1993.