Recent research (A. Ducci and M. Yianneskis M., Vortex tracking and mixing enhancement in stirred processes, AIChE J 53(2) (2007), p. 305-315) has shown that mixing times in reactors stirred by radial impellers can be reduced by 20-30% when feed insertion is made in the vortex core of precessional flow structures, denoted as macro-instabilities (MIs). The aim of the present work is to investigate the interaction between large-scale flow structures, such as MIs and trailing vortices, and assess to which extent local mixing might be affected by their combined activity. Proper orthogonal decomposition (POD) analysis was employed to identify and characterise particle image velocimetry (PIV) data of the different flow structures. It is shown that the combined presence of the trailing vortex and MI structures in the impeller vicinity results in energy levels that are substantially enhanced and thus locations showing further promise for feed insertion and mixing enhancement may be identified. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.