Although most of the work reported oil two-phase flows are limited to small pipe diameters, two-phase flow in large risers are increasingly being encountered in the petroleum and nuclear industries. In the present work, a wire mesh sensor was employed to obtain void fraction and bubble size distribution data and visualizations of steam/water flow in a large vertical pipe (194 mm in diameter) at 46 bar. For comparison purposes, measurements were made at similar phase velocities and physical properties to a dataset for nitrogen/naphtha flow in a similar-sized riser. There exist significant differences between both sets of data. Churn-turbulent flow is observed in the present work instead of slug flow, and this differs from the intermittent and semi-annular flow patterns reported for nitrogen/naphtha data. The mean void fraction of the nitrogen/naphtha data is higher than that of the present steam/water data due to the differences in purity in the liquid phases. Furthermore, core peak distributions are observed for the present work in contrast to the flatter profiles deduced for the nitrogen/naphtha using a power law relationship. (c) 2007 Elsevier Ltd. All rights reserved.