Drag coefficient correlations for bubbles in buoyancy-driven two-phase flows have generally been derived from data on low-viscosity media and within the bubbly flow regime in a number of applications, e.g. evaporative crystallizers, there is a need to extend this correlation to higher viscosity flows and slug regimes In this paper, the momentum interaction in gas-liquid vertical channel flow has been studied experimentally over a wide range of void fractions using a circulation loop facility where the buoyancy is the only driving force for liquid circulation. A model for the drag in gas-liquid buoyant flows has been developed, and is applicable for a wide range of viscosity and void fractions. (C) 2009 Elsevier Ltd. All rights reserved