The construction of a gas-fluidized bed of annular cross-section, contained between two vertical cylinders, the internal one of which can be made to rotate at selected speeds, is described. The system was used to determine the apparent viscosity of a fine powder fluidized homogeneously by ambient nitrogen over a range of expansion conditions. The results revealed pseudo-plastic-type behavior, the measured apparent viscosities decreasing markedly with increasing shear stress. (c) 2008 Elsevier Ltd. All rights reserved.