The flow field around air bubbles rising in aqueous polyacrylamide (PAAm) solutions was studied using a particle image velocimetry (PIV) system. This flow was analyzed in the vicinity of the critical bubble volume where the discontinuity of the terminal bubble velocity occurs. It was found that the flow configuration changes drastically below and above the critical bubble volume. The flow just below the critical bubble volume shows an upward flow at the front and back of the bubble with a symmetrical vortex around the bubble. The flow field obtained for a volume just above the critical one shows the appearance of the so-called negative wake behind the bubble. Additionally, it was found that the container walls affect significantly the magnitude of the terminal velocity as well as the velocity jump. However, the critical volume at which the velocity jump appears does not change for different container sizes. (C) 2003 Elsevier Science B.V. All rights reserved.