Mechanisms of defluidization are investigated by discrete element method simulation. The particles used are 0.2, 0.5, 0.8, 1.2 and 1.5 mm in diameter and 2650 kg/m(3) in density. An artificial interparticle force is imposed on the particles in order to reproduce defluidization. The global bed behaviour is related to the mobility of individual particles, enabling a quantitative description of the degree of defluidization to be obtained. The effect of gas velocity on the degree of defluidization is analysed. New results of collision dynamics, such as particle contact time and particle speed, are obtained for conditions where interparticle cohesive force cannot be neglected. A new criterion for defluidization is developed based on analysis of normal forces acting on the particles. (C) 2003 Elsevier Ltd. All rights reserved.