The interfacial area transport of vertical, upward, steam-water two-phase flows in a vertical annular channel has been investigated. The inner and outer diameters of the annular channel were 19.1 and 38.1 mm, respectively. The test section had a 2845 mm heated section followed by a 1632 mm unheated section. Fifty seven experimental conditions were selected, which cover bubbly, cap-slug, and churn-turbulent flows. Each one of flow conditions was obtained by achieving different inlet sub-cooling temperatures, liquid velocities, wall heat flux or system pressures. The local flow parameters, such as void fraction, interfacial area concentration, and bubble interface velocity, were measured at different radial positions for the five axial locations. The radial and axial evolutions of local flow structure were interpreted based on presence of wall superheat, wall nucleation, bulk condensation and evaporation, bubble sizes, coalescence and break-up mechanisms. The measured data can be used for both the assessment of the bubble coalescence/breakup models and the development of closure models for computational fluid dynamics. (C) 2012 Elsevier Ltd. All rights reserved.