Electrical resistance tomography (ERT), which is a non-intrusive flow visualization technique, was used to investigate the solid-liquid mixing in an agitated tank equipped with a top-entering axial-flow impeller. The signals obtained from eight ERT planes were utilized to reconstruct the tomograms by using the linear back projection algorithm. The ERT measurements were correlated to solid concentration profiles by which the degree of homogeneity was quantified. In this study, the effect of important parameters such as impeller type (Lightnin A100, A200, A310, and A320 impellers), impeller speed (250-800 rpm), impeller off-bottom clearance (T/5-T/2, where T is the tank diameter), particle size (210-1500 mu m), and solid concentration (5-30 wt%) on the degree of homogeneity were explored. The results showed that the degree of homogeneity in the solid-liquid mixing was improved with increasing the impeller speed. However, after reaching the maximum achievable homogeneity, further increase in impeller speed was not beneficial but might be detrimental. Hence, the measurement of the optimal impeller speed as a function of operating conditions and design parameters has vital role in achieving maximum homogeneity in a solid-liquid mixing system. (C) 2009 Elsevier Ltd. All rights reserved.