Steady-state and transient experiments of water-gas, oil-water and oil-water-gas multiphase flows were conducted on a transparent I I in long, 15 cm, diameter, inclinable pipe using kerosene, tap water and nitrogen. The pipe inclination was varied from 0degrees (vertical) to 92degrees and the flow rates of each phase were varied over wide ranges. Fast-acting valves enabled the trapping of the fluid flowing in the pipe, which in turn allowed for the accurate measurement of the absolute volumetric fraction (holdup) of each phase. A nuclear densitometer, as well as ten electrical probes at various locations along the pipe, provided additional steady-state and transient measurements of the holdup. A total of 444 tests were conducted, including a number of repeated tests. Bubble, churn, elongated-bubble, slug, and stratified/stratified-wavy flows were observed for water-gas and oil-water-gas flows, while dispersed/homogeneous, mixed/semi-mixed and segregated/semi-segregated flows were observed for oil-water flows. Extensive results for holdup as a function of flow rates, flow pattern and pipe inclination are reported, and the various techniques for measuring holdup are compared and discussed. The flow pattern and shut-in holdup are also compared with the predictions of a mechanistic model. Results show close agreement between observed and predicted flow pattern, and a reasonable level of agreement in holdup. (C) 2003 Elsevier Science Ltd. All rights reserved.