Comminution within rod mills has been investigated by izse of a matrix analysis of breakage processes. A para,~ meter, the number of breakage stages, has been introduced and cornfninut-ion within all rod mills has been expressed in terms of the1-number of breakage stages- and a function which des-- cribes' the feed rate, mill diameter and mill speed.' The effect of variation in feed rate, feed sizing analysis and mill speed on the number of breakage stages-is-,- discussed.