Repeated dehydriding/hydriding cycles of ball-milled nanocrystalline MgH(2) powders were carried out in a Sievert's-type apparatus. The microstructural changes during cycling were characterized by X-ray diffraction. Using the convolutional multiple whole profile fitting procedure, several microstructural parameters, i.e. the average grain size and the grain-size distribution of the as-milled and treated samples were determined. Based on the shrinking core model, the significant difference between the microstructures obtained after the first and repeated cyclings was explained. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.