Electrical capacitance tomography (ECT) is a new technology that is used to visualise the internal behaviour of industrial processes comprising dielectric components. A particular difficulty with capacitance measurement for tomography is that the stray capacitance to earth of an ECT sensor is large (~150 pF) compared with the interelectrode capacitance to be measured (usually <0.5 pF), and the measurement range is wide (0.01–2 pF). The authors present a highly sensitive AC-based capacitance measuring circuit, which has been specially designed for this purpose, and a PC-based ECT system with this circuit. Experimental results show that this ECT system can measure as small as 0.01 pF capacitance with 0.0002 pF error, and could collect tomographic image data at more than 100 frames per second.