An experimental and theoretical study of the agglomeration phenomenon which causes destabilization of certain low and high temperature fluidized beds was performed. A theoretical model was proposed to determine the conditions under which defluidization occurs in fluidized beds in which cohesion forces between granules arise due to the presence of sticky fluids and/or high temperatures. Bonding mechanisms between particles such as solid-liquid bridges, viscoelastic flattening and high temperature sintering were all considered. The model, which predicts breakup of aggregates by bubble motion, was compared to limiting fluidization-defluidization (quenching) experiments performed by the authors and others. An experimental method to measure surface softening of small particles heated to high temperatures was developed by using a dilatometer to measure the surface viscosity of the particles from rate of deformation data. Experimental methods to determine the minimum sintering temperatures of a variety of granules were also presented. Lastly, experiments were performed to study the dynamic strength of a liquid bridge between two spheres coated with a liquid and moving away from one another, It was shown that the strength of the dynamic bridge was at least one order of magnitude larger than the corresponding strength of the static bridge between the two spheres. This result accounts for the relatively high gas velocities necessary to keep a bed of sticky particles in continuous fluidization.