The kinetics of solid-state polymorphic transition of the stable polymorph of glycine (Form gamma) to an unstable Form a were studied using a planetary ball mill to demonstrate the effect of mechano-chemical processing. The kinetics were analyzed in terms of operating variables, i.e. revolution speeds, the number of balls, mass of sample, materials of jar and balls, and grinding time. The rate of the transition was well correlated with the two-dimensional Avrami equation, and the apparent rate constants empirically determined were correlated with the above-mentioned variables. For the transition in a stainless steel jar and balls, the changes of polymorphic compositions were successfully correlated over the ranges of experimental conditions examined. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.