The present paper investigates void fraction and pressure drop characteristics of vertical air-water down flow through millichannels (0.83 <= Eotvos Number <= 20.6). Experiments have revealed the hydrodynamics to be a function of tube diameter and phase velocities with the functional form being different for different flow patterns. Accordingly, separate mechanistic models have been proposed to predict the aforementioned parameters for bubbly, slug, falling film and annular flow distributions. The proposed analysis have been validated with experimental results of the present study and those reported in literature. (C) 2017 Elsevier Ltd. All rights reserved.