A new method to predict the void fraction in annular two-phase flow in macroscale and microscale channels is presented. The underlying experimental database contains 2673 data points collected from 29 different literature studies for 8 different gas-liquid and vapor-liquid combinations (water-steam, R410a, water-air, water-argon, water-nitrogen, water plus alcohol-air, alcohol-air and kerosene-air), for tube diameters from 1.05 mm to 45.5 mm and for both circular and non-circular channels. The new prediction method is strongly simplified with respect to most existing correlations, as it depends only on vapor quality and the gas to liquid density ratio and reproduces the available data better than existing prediction methods. Importantly, this study shows that there appears to be no macro-to-microscale transition in annular flows, at least down to diameters of about 1.0 mm. (C) 2012 Elsevier Ltd. All rights reserved.