It is shown that the presence of two active zones in a grinding mill leads to apparent non-first-order breakage of the top feed size in batch grinding. The equations to describe the kinetics of the breakage of the top size and of the total size distribution are obtained, and an analytical solution is given. Parametric analysis shows the influences of the relative magnitudes of the two active zones, the rates of breakage and the rate of mass transfer between the two active zones on the mill product. The concepts may be applicable to any type of mill where two active zones can exist.