The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; gamma-Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d(90) of 37 mu m to 2.9 mu m compared to planetary ball milling (30.2 mu m) and single ball milling (10.5 mu m). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired alpha-Alumina resulting in a notable decrease in surface area from 136.6 m(2)/g to 82.5 m(2)/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).