The behaviour of gas-particle interaction in a fluidized bed depends strongly on the size of the particles being fluidized. Fluidization characteristics of macro-sized particles, from several tens of microns to several millimetres, are well described by the Geldart [1973. Types of gas fluidization, Powder Technology 7, 2851 classification. There is evidence that fluidization characteristics of nano-sized particles may be analysed by analogies to those of macro-sized particles. The aim of this paper is to look at whether the mechanism resulting in cohesive behaviour for macro-sized particles may be extended to nano-sized particles. Degussa Aerosil R974 powder, with a primary particle size of 12 nm, was fluidized using nitrogen in a cylindrical vessel, 50-mm-id and 900 mm in height. Characteristics of incipient fluidization are analysed in relation to variations in the initial packed bed conditions. Bed collapse experiments were performed and the results are used for assessing fluidization characteristics of the particles. It was found that nano-sized particles possess characteristics of both Group A and Group C of Geldart classification. Analysis shows that fluidization characteristics of nano-sized particles may be bridged to those of macro-sized particles through particle aggregation. (c) 2007 Elsevier Ltd. All rights reserved.