The flow structures in downers have been investigated by a micro-video and by analysis of local voidage signals in this work. The experiments of micro-video action shot were performed in a downer with an internal diameter of 0.09 m. The local voidage signals were collected in a downer with an internal diameter of 0.285 m. The micro-video action shot showed that there were particle-clustering phenomena in the downer and the clusters existed mainly in the form of floc and stick. Through wavelet analysis of local voidage signals, the cluster size and frequency were obtained. However, the probability density distributions of local voidage signals confirmed that the clusters in the downer were unstable and could not form a stable phase. (c) 2005 Elsevier B.V. All rights reserved.