An experimental study is conducted to determine the effect of different types of nanoparticles on the gas fluidization characteristics of nanoparticle agglomerates. Taking advantage of the extremely high porosity of the bed, optical techniques are used to visualize the flow behavior, as well as to measure the sizes of the fluidized nanoparticle agglomerates at the bed suface. Upon fluidizing 11 different nanoparticle materials, two types of nanoparticle fluidization behavior, agglomerate particulate fluidization (APF) and agglomerate bubbling fluidization (ABF), are observed and systematically investigated. A simple analytical model is developed to predict the agglomerate sizes for APF nanoparticles, and the results agree fairly well with the optical measurements. Using the Ergun equation, the experimentally measured pressure drop and bed height, and the average agglomerate size and voidage at minimum fluidization predicted by the model, the minimum fluidization velocities, for APF nanoparticles are calculated and also agree well with the experimental values. Other important fluidization features such as bed expansion, bed pressure drop, and hysteresis effects, and the effects of the primary particle size and material properties are also described. (C) 2005 American Institute of Chemical Engineers.