Experimental investigations on the comminution of gibbsite in four different types of mills are presented, These include tests of dry grinding in a tumbling ball mill, a shaker bead mill and an air jet mill as well as tests of wet grinding in a tumbling ball mill and in a stirred bead mill, A quantitative characterization of the size and the shape of the ground product has been made in order to analyze the effect of the grinding stress on the properties of a complex agglomerate crystal such as gibbsite. The analysis allows us to determine the fragmentation route occurring in the mills. This has led to the identification of two fragmentation paths, involving different phenomena: rupture of joints, chipping and breakage according to the nature of the main stress (attrition or impact) imposed during comminution processes. (C) 2001 Elsevier Science B.V. All rights reserved.