The fluidization conditions in a fluidized bed can vary dramatically, from bubbling to circulating fluidization. Considering the dielectric properties and the rapid movement of the solids in fluidized beds, electrical capacitance tomography (ECT) is particularly suitable for measuring the concentration and distribution of solids in fluidized beds. This paper reports the use of a square ECT sensor to investigate the behaviour of fluidized beds, showing the unique advantages of using ECT in measuring cross-sectional concentration profiles of solids. Different fluidization regimes manifest different characteristics, in terms of the concentration distribution of solids.