The gas-solids flows in the dense pneumatic conveying exhibit many interesting features related to their three-dimensional and unsteady nature. The dominant characteristic in this type of flows is the appearance of discrete flow instabilities commonly known as 'slugs' and 'plugs'. These are investigated by two techniques: a high-speed video camera and a twin-plane electrical capacitance tomography (ECT), and the results obtained are discussed. Data extracted from tomographic measurements can be processed and presented in several different ways to enable a better understanding of the flow behaviour. Several examples are given, including body shape type information, internal structure of the flow instabilities, their propagation velocity and frequency characteristics. A short case study, focused on measurements of bidirectional flow in a vertical channel, is presented.