The breakage characteristics of mixtures of materials were investigated in a laboratory ball-race mill. A study of the grinding kinetics of the components in a binary mixture showed that the breakage rate of one material was affected by the presence of the other material. The breakage rate of the relatively; weaker material increased and, conversely, the breakage rate of the stronger material decreased. The degree of the change in the breakage rate was dependent on the blending ratio and the relative difference in the "strength" of the component. Because there was a consistent pattern, an empirical relationship for this variation was developed. The breakage distribution values of the stronger material were found to be independent of whether the material was ground alone or as a component in a binary mixture. However, the breakage distribution values of the weaker material appeared to change, producing more fines when ground in a binary mixture. Thus, the proportion of the stronger material in the coarser size fractions of the product is greater after mixing the two feed materials and grinding them than after grinding the two materials separately and mixing the products. A similar variation in the breakage rates was observed for a three-component system. The breakage rate decrease of the strongest material and the breakage rate increase,of the weakest material were very close to those observed for a binary mixture. However, the breakage rate of the intermediate hardness material stayed essentially the same, even though a slight decrease was expected.