Accurate prediction of the interfacial area concentration is essential to successful development of the interfacial transfer terms in the two-fluid model. Mechanistic modeling of the interfacial area concentration entirely relies on accurate local flow measurements over extensive flow conditions and channel geometries. From this point of view, accurate measurements of flow parameters such as void fraction, interfacial area concentration, and interfacial velocity were performed by a multi-sensor probe at three axial locations as well as seven radial locations in vertical downward bubbly flows using a 25.4 mm-diameter pipe. In the experiment, the superficial liquid velocity and the void fraction ranged from -1.25 to -3.11 m/s and from 1.61% to 21.0%, respectively. (C) 2004 Elsevier Ltd. All rights reserved.