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Although, most of the work reported on 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 (194mm in diameter) at 46bar. For comparison purposes, measurements were made at similar phase velocities and physical properties to the data of Omebere-Iyari et al. (2006), which is for the flow of a nitrogen/naphtha mixture 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 which differs from the intermittent and semi-annular flow patterns reported by Omebere-Iyari et al. (2006). The mean void fraction of the nitrogen/naphtha data is higher that of the present steam/water data. On examination of radial void fraction profiles, core peak distributions are observed for the present work in contrast to the wall peak profiles predicted for the data of Omebere-Iyari et al. (2006) using a power law relationship.