A new database of steam water flow in a vertical annulus is developed in the Multiphase Thermo-fluid Dynamics Laboratory (MTDL) at the University of Illinois at Urbana-Champaign. The focus of the dataset is on the two-phase phenomena of condensing, flashing, and saturated flow in a vertical channel. The 2 m annular channel with inner and outer diameter of 19.1 mm and 38.1 mm respectively is located downstream of an internally heated annular section. Five measurement ports in this unheated section provide measurement of pressure and local measurements of void fraction, interfacial area concentration, gas velocity, Sauter mean diameter, and liquid temperature. The dataset was designed for validation of one-dimensional system codes and multi-dimensional computational fluid dynamics codes. Detailed local measurements of three representative cases are shown to highlight the complexity of local two-phase flow phenomena observed along the test section. A parametric analysis is also conducted to show the sensitivity of system parameters on the axially evolving two-phase flow. The focus of the adiabatic steam-water flow fills an important gap in validation data between the simplified hydro-dynamic data of air-water flow and the complex thermo-fluid dynamic data of boiling flows. (C) 2019 Elsevier Ltd. All rights reserved.