Continuous-flow mixing of pseudoplastic fluids possessing yield stress is a complex phenomenon exhibiting non-ideal flows within the stirred vessels. Electrical resistance tomography (ERT), a non-intrusive technique, was employed to measure the mixing time in the batch mode while dynamic tests were performed to study the mixing system in the continuous mode. This study attempts to explore the effects of the operating conditions and design parameters on the ratio of the residence time (tau) to the mixing time (theta) for the continuous-flow mixing of non-Newtonian fluids. To achieve these objectives, the effects of impeller types (four axial-flow impellers: A310, A315, 3AH, and 3AM; and three radial-flow impellers: RSB, RT, and Scaba), impeller speed (290-754 rpm), fluid rheology (0.5-1.5%, w/v), impeller off-bottom clearance (H/2.7-H/2.1, where H is the fluid height in the vessel), locations of inlet and outlet (configurations: top inlet-bottom outlet and bottom inlet-top outlet), pumping directions of an axial-flow impeller (up-pumping and down-pumping), fluid height in the vessel (T/1.06-T/0.83, where T is the tank diameter), residence time (257-328 s), and jet velocity (0.317-1.66 ms(-1)) on the ratio of tau to theta were investigated. The results showed that the extent of the non-ideal flows (channeling and dead volume) in the continuous-flow mixing approached zero when the value of tau/theta varied from 8.2 to 24.5 depending on the operating conditions and design parameters. Thus, to design an efficient continuous-flow mixing system for non-Newtonian fluids, the ratio of the residence time to the mixing time should be at least 8.2 or higher. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.