The mean rise velocity of bubble swarms ascending in shear-thinning fluids was experimentally measured in a rectangular bubble column. Great care was taken to produce nearly mono-dispersed bubble swarms and to use shear-thinning fluids with negligible elastic effects. In this manner, it was possible to isolate the effect of the hydrodynamic interaction between bubbles in the column caused by the thinning behavior of the liquid. It was found that the mean rise velocity of the bubbles was larger than that of an individual bubble, in accordance with previous studies. The magnitude of the swarm velocity was found to be greatly influenced by the appearance of bubble clusters. The bubble clusters, which appeared for certain values of the flow index and bubble diameter, were found to have a very different structure from those observed in Newtonian liquids. Furthermore, it was found that the appearance of clusters produced a dramatic increase of the agitation within the column. A set of conditions was identified for the appearance of bubble clusters in shear-thinning inelastic bubbly columns. (C) 2010 Elsevier B.V. All rights reserved.