For a successful application of fluidized beds in chemical reactions and solids circulation, the boundary of regime of operation such as bubbling and slugging regimes, needs to be clearly defined. This study provides a method for determining the onset of fluidized bed regimes using a two-plane electrical capacitance tomography (ECT) sensor. The method involves computation and analysis of standard deviations of the solids fraction recorded at each plane of the ECT sensor for different superficial gas velocities. The experimental study is based on two different samples of 100-550 mm glass particles and one sample of 150-450 mm limestone particles. The results show that the onset of bubbling is determined when a bubble is first observed in the upper plane. The onset of slugging is obtained at the peak of the difference in the solids fraction fluctuation between the two planes, which is determined at the point where the rates of increase in the fluctuations are the same in both planes. The method developed in this study provides a means of obtaining accurate superficial gas velocities at the onset of slugging in fluidized beds. (C) 2017 The Authors. Published by Elsevier B.V.