In view of the importance of two-group interfacial area transport equation and lack of corresponding accurate downward flow database that can reveal two-group interfacial area transport, a systematic database for adiabatic, air-water, vertical downward two-phase flow in a round pipe with inner diameter of 25.4 mm was collected to gain an insight of interfacial structure and provide benchmarking data for two group interfacial area transport model. Four-sensor conductivity probes were used to measure the local two phase flow parameters at high data sampling frequency to ensure the accuracy. Axial development of local flow parameter profiles including void fraction, interfacial area concentration, and Sauter mean diameter were presented. Drastic inter-group transfer of void fraction and interfacial area as well as the wall peaked interfacial area concentration profiles were observed and discussed. (C) 2016 Elsevier Ltd. All rights reserved.