Among electrical tomography techniques, electrical capacitance tomography (ECT) has been the subject of extensive recent research due to its non intrusive and non-invasive nature. It is used for obtaining information about the distribution of the contents of closed pipes or vessels by measuring variations in the dielectric properties of the material inside the vessel. An experimental study was conducted by producing two-phase bubble flow regime in a vertical online ECT column. This experimental study measures the void fraction in a two-phase bubble flow regime by using ECT and differential pressure (P). Differential pressure is also a simple and reliable approach. The experiments conducted for this study used compressed air as the gas phase and deionised water as a liquid phase. On the variation in superficial gas velocity from 0.0021 to 0.03 m/sec, and superficial liquid velocity from 0 to 0.034 m/sec, some different kinds of sub-bubble flow regimes were observed within the column. The experimental results show the influence of gas velocity on the void fraction in an increasing manner. The raw data obtained from ECT was also compared with the simulated data which shows a good agreement to each other. © 2011 IEEE.