Degussa Aerosil R974 powder, with a primary particle size of 12 nm, was fluidized using nitrogen in a glass vessel of 46 turn square cross-section. Images of nanoparticle aggregates were recorded with a high-resolution digital CCD camera. The settling velocities of the nanoparticle aggregates were obtained through PIV (particle image velocimetry) analysis and manual tracking of individual aggregates. The results enabled an assessment of the applicability of the Richardson-Zaki equation to nanoparticle fluidization. The internal structure and formation mechanisms of nanoparticle aggregates formed under fluidization conditions are discussed in the light of the results obtained. (c) 2006 Elsevier Ltd. All rights reserved.