Abstract: Mixing is a common process in many industrial applications, but due to its complexity theoretical approaches are very limited. This study described the use of Electrical Resistance Tomography (ERT) in characterizing and quantifying gas-liquid mixing. Tomography is a robust and powerful technique to visualize 2D and 3D conductivity image in a stirred agitating vessel using multiple planes of periphery spaced sensors. The experimental work was carried out in a fully baffled agitated vessel with a diameter of 40 cm, 56 cm height and equipped with ITS P2000 ERT. On the basis of the reconstructed pixel of conductivity from multi-plane sensors arrangement, the affected parameters of mixing can be measured. A MATLAB® code was developed to construct surface plots of gas holdup. The gas concentration tomograms and holdup surface plots successfully mapped the gas distribution within the vessel.