Gas-Newtonian liquid two-phase flows (TPFs) are presented in several industrial processes (e.g. oil-gas industry). In spite of the common occurrence of these TPFs, the understanding of them is limited compared to single-phase flows. Various studies on TPF focus on developing empirical correlations based on large sets of experimental data for void fraction, which have proven accurate for specific conditions for which they were developed limiting their applicability. On the other hand, few studies focus on gas-non-Newtonian liquids TPFs, which are very common in chemical processes. The main reason is due to the characterization of the viscosity, which determines the hydraulic regime and flow behaviours of the system. The focus of this study is the analysis of the TPF (slug flow) for Newtonian and non-Newtonian liquids in a vertical pipe in terms of void fraction using computational fluid dynamics (CFD) and comparing this directly with experimental measurements and empirical relationships found in literature. A vertical tube of 3.4 m with an internal diameter of 0.1905 m was used. The two-phase CFD model was implemented in Star CCM+ using the volume of fluid (VOF) model. A relatively good agreement was found between the experimental measurements, the CFD results and the empirical relationships. In terms of void fraction for Newtonian and non-Newtonian liquids, the empirical correlations perform much worse than the CFD simulations, errors of 48 and 25%, respectively, against the experimental data. This shows that CFD can be used to predict void fraction relatively well for comparison against empirical correlations and they can be used for design and scale-up processes. (c) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.