Uniform and efficient particulate-liquid contact is of critical importance in solid-liquid systems. Typically, complete homogeneity of the particulate suspension is desired, but not achieved. In mechanically agitated contactors, the critical impeller speed for off-bottom suspension, N-js, and cloud height are used to assess solids distribution across the axial axis as an indicator of suspension efficiency. Further, various, but inconsistent, mixing indices have been proposed previously to quantify homogeneity. In this paper, the definition of mixing indices is extended to describe homogeneity in terms of the average of the standard deviation in solids concentration across the axial and radial directions as well as across the overall reactor. These mixing indices are quantified using data collected using electrical resistance tomography (ERT) in which an unstructured mesh matching the vessel geometry was used during reconstruction. The average standard deviation of solids concentrations in each volume element across four planes and seven annuli provided the axial (MIz) and radial (MIr) mixing indices. Combined data across the 28 regions provided the overall mixing index (MIo). These indices were compared across three particle size distributions, three solids concentrations and five impeller speeds. The data demonstrate inhomogeneity in solids concentration across both the radial and axial directions as a function of operating conditions. For the coarse and medium particle fractions (600-850 um and 425-600 mu m), MIo is dominated by MIz whereas MIr dominated MIo for the fine particle fraction (106-212 mu m). In all cases, homogeneity of suspension decreases with decreasing solids concentration across the range 5-20% by volume. Homogeneity improves with increasing impeller speed, up to approximately 0.8N(js) for the medium and coarse fractions. Little dependence of mixing index on impeller speed was observed for the fine fraction across the range 0.5N(js) to 1.4N(js). The limited dependence of homogeneity on impeller speeds above 0.8N(js) suggest significant energy savings can be achieved without loss of suspension by operating at impeller speeds below N-js. (C) 2011 Elsevier Ltd. All rights reserved.