This paper presents the results of the characterization of a new method for quantification of the micromixing efficiency in a viscous medium in stirred tank reactors. The used method is inspired by a chemical system based on two competitive reactions, the iodide-iodate reaction system (Villermaux-Dushrnan method), whose products selectivity is a measure of the mixing efficiency. The approach proposes a new protocol based on a phosphate buffer and the use of HydroxyEthylCellulose (NEC, 720, 000 g/mol) as an inert viscosifying agent, exhibiting various interesting properties (no disturbance of the spectrophotometric measure, low cost, low required amount, eco-friendly product). This agent is almost Newtonian up to 0.25 wt% in water and becomes non-Newtonian above 0.5 wt%. By means of its use, the viscosity of the reactive medium can be raised by around two orders of magnitude while adding less than 1 wt% HEC to the water solution. Preliminary micromixing results of moderate viscous solutions (50 mP as with NEC 0.5 wt%) in a stirred tank reactor are presented. (C) 2014 Elsevier Ltd. All rights reserved.