The liquid mixing time in solid-liquid systems is difficult to determine by quantitative and non-intrusive methods, particularly at high solid loadings. Three methods are considered for the post-processing of raw conductivity data collected by electrical resistance tomography (ERT) in a baffled stirred tank. A novel approach is suggested to overcome the limitations of the section-averaged conductivity evaluations and the problems associated with the solids disturbances affecting the local conductivity data. The new approach, which is based on the classic post-processing method devised for the mixing time determination in optically accessible stirred tanks by planar laser-induced fluorescence, is successfully applied to the ERT data collected either under single-phase or two-phase conditions.