Stirred tank reactors are one of the most important and common pieces of equipment used in speciality, pharmaceutical, and agrichemical processes. It is also typical for these to be operated at large aspect ratios; however, there is very little information in the open literature about mixing times in vessels with aspect ratios greater than one. This paper aims to provide new information in this area that will enable the design of better reactors. Electrical resistance tomography is used to monitor the mixing time of systems with aspect ratios higher than one. The mixing time has been measured on vessels of 0.914 and 0.610 m diameter with Cowles disc, Rushton turbine and mixed flow type impellers to aspect ratios of 2. The current correlation of choice, by Grenville and Nienow (2004), has been compared with the results and found to under predict the mixing time at aspect ratios greater than one. The exponent on the H/T term has been explored and it has been found that this varies with agitator type, this information has never been shown before. The affect of adding a second impeller on the mixing time and flow pattern is also investigated. Adding a second Rushton turbine creates zoning in the vessel which impedes the mixing; this can be visualised using electrical resistance tomography. (C) 2011 Elsevier Ltd. All rights reserved.