In this study, the residence time distribution of a CSTR was measured and analysed for variations in CSTR feed geometry, impeller speed and inlet flow rate. The measured residence time distributions were also used to characterise the degree of plug flow behaviour and short-circuiting. The data showed that the performance of a CSTR could not be evaluated using the ratio of the mean residence time to the batch-mode mixing time (tau/t(M)), and that this approach can lead to process over-design and excessive energy requirements. Instead, it was shown that all process parameters could be correlated using the ratio of the inlet jet momentum to the impeller discharge momentum. The prediction of the degree of short-circuiting and plug flow in the CSTR could be used to improve process control. In addition, the results can be used to identify whether the inlet position or diameter of an existing CSTR should be modified to improve process performance. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.