Mixing characteristics of an aerated coaxial mixer composed of an anchor and a central impeller was investigated using the non-invasive flow visualization technique called electrical resistance tomography (ERT). Corn syrup solutions with different viscosity were used as the viscous Newtonian fluids. Two coaxial configurations were considered: the anchor PBD (a pitched blade downward pumping impeller) and the anchor PBU (a pitched blade upward pumping impeller). In this study, the effects of central impeller types, speed ratios (central impeller speed/anchor speed), rotation modes, gas flow rates, and viscosity on the mixing time and power uptake were explored. It was found that in the presence of gas, the PBU-anchor coaxial combination in co-rotating mode exhibited shorter mixing times and lower power consumption than the PBD-anchor. Experiments demonstrated that the effect of aeration on the mixing time was a function of hydrodynamic regimes occurring in the tank. Using the response surface method, an effort was made to develop a quadratic model as a function of central impeller speed, anchor speed, gas flow rate, and viscosity for predicting the mixing time. Three-dimensional response surfaces were plotted to understand the main and interaction effects of these factors. (C) 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.