A study that focussed on the effects on agglomeration of changing the rotational speed of a vertical-axis high shear mixer is reported. The design of the mixer was such that, at high impeller speeds, the power input was high. The agglomeration behaviour was found to vary greatly with impeller speed. At low impeller speed, the extent of size enlargement increased with power input. At high impeller speed, the extent of size enlargement was low relative to the large power input. From examination of the changes in size distributions of the granules and of granule morphology, it was concluded that, at high speed, size enlargement was limited by granule breakage. The observation is important as it may provide an improved means of controlling the: process.