The pulverization, expansion characteristics of La0.6Y0.4Ni4.8Mn0.2 and their influences in horizontal and vertical thin-wall reactors were investigated. Granulometric and SEM results revealed particle decay history and the transform from polydisperse to monodisperse state, then the reactor strain in both longitudinal and tangential directions was tested with packing fractions being 20-58.5vol%. Results indicated that the longitudinal strain was less than the hoop strain; both increased with cycle numbers, initial packing fractions and hydrogen contents, while decreased from the lower to the upper positions. Horizontal reactors exhibited less strain than vertical ones; the former mainly bended and swelled while the latter bulged angularly. The strain increments were approximate linear along with cycles, while the absorption strain increments grew exponentially along with hydrogen contents. It is suggested that the optimal hydrogen contents for La0.6Y0.4Ni4.8Mn0.2 in the fully packed thin-wall reactors be less than 0.6 mol H/mol alloy whereas the packing fractions should not exceed 35 vol% for the case of hydrogen contents around 1.0 mol H/mol alloy. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.