Gas-liquid annular flow through a vertical circular pipe is well-understood and detailed phenomenological models exist in the literature. In the present work, the case of flow through a diverging section is studied experimentally and theoretically. Experiments have been carried out in air-water flow through a vertical diverging pipe section with a diameter ratio of 1.5 and 2.0. Pressure profiles have been recorded upstream, across and downstream of the diverging section for the cases of sudden expansion and gradual expansion with included half-angles of 80 and 15 for the diverging section. These show that the pressure variation is characterized by a strong pressure recovery downstream of the expansion which is in turn influenced by the smoothness of the expansion and the interfacial friction. The non-dimensionalized pressure loss across the expansion, which has been determined by extrapolating the pressure variations upstream and downstream, is found to vary systematically with the diameter ratio, the angle of the expansion and the ratio of superficial liquid-to-gas Reynolds numbers. A method, which uses the equilibrium annular flow model and an empirically determined expansion pressure loss coefficient, has been developed for the prediction of the pressure variation across the diverging section. (C) 2014 Elsevier Ltd. All rights reserved.