Flow structures and phase distributions in two- and three-phase stratified and slug flows were studied with a fast-response X-ray tomography system. A large number of experiments were carried out in a 25 m long near-horizontal pipeline with an internal diameter of 100 mm. A dense gas, SF6, was used to simulate the gas phase in high-pressure gas-condensate pipelines. Detailed measurements of the flow were made with the X-ray system at various superficial velocities, two different pressures (4 and 7.2 bara, giving gas densities of 24 and 42 kg/m(3), respectively), and three different inclination angles: 0, +1 and +5 degrees. Both qualitative observations and quantitative measurements were obtained which illustrate the different types of interface structure and fluid flow behaviour. The detailed measurements have also been used to obtain a more reliable correlation for predicting the fraction of gas bubbles entrained in the liquid layer in turbulent stratified gas-liquid flows. The measurements in three-phase slug flows reveal interesting phenomena regarding the interactions of the oil-water and gas-oil interfaces. Depending on the flow conditions, the oil-water interface can sometimes decouple from the gas-liquid interface, leading to oil-dominated slugs flowing on top of a water layer of nearly constant thickness. (C) 2014 Elsevier Ltd. All rights reserved.