Defluidized zones often appear on the distributor plates of liquid-solid fluidized beds. They can lead to hot spots, the formation of undesirable side products or the degradation of products or reactants. In some cases, a solid residue forms and plugs the distributor. Two different techniques were developed to detect defluidized zones. The first technique uses a specially designed collision probe to monitor local particle motion. The second technique is aimed at the on-line detection of defluidized zones in industrial bioreactors. It uses local bed conductivity fluctuations. Defluidized zones were measured in beds of 3 or 5 mm diameter glass beads fluidized by an aqueous saline solution. Special experiments established the importance of horizontal liquid flow and distributor plate roughness on the formation of defluidized zones. A model describes how a defluidized zone can be eliminated. It considers that a defluidized zone is broken by the drag force on its particles of downward and sideways liquid flow. This liquid flow is induced by suction from the liquid jets issuing from the distributor holes. The resulting drag force is resisted by friction between particles or between particles and the distributor surface.