Summary Oil/gas pipe flows are expected to exhibit significantly different behavior at high oil viscosities. Effects of high-viscosity oil on flow pattern, pressure gradient, and liquid holdup are experimentally observed, and differences in flow behavior of high- and low-viscosity oils are identified. The experiments are performed on a flow loop with a test section of 50.8-mm ID and 18.9-m-long horizontal pipe. Superficial liquid and gas velocities vary from 0.01 to 1.75 m/s and from 0.1 to 20 m/s, respectively. Oil viscosities from 0.181 to 0.587 Pa·s are investigated. The experimental results are used to evaluate the performances of existing models for flow pattern and hydrodynamics predictions. Comparisons of the data with the existing models show significant discrepancies at high oil viscosities. Possible reasons for these discrepancies are carefully examined. Some modifications are identified and implemented to the closure relationships employed in the Zhang et al. (2003) model. After these modifications, the model predictions provide better agreement with experimental results for flow pattern transition, pressure gradient, and liquid holdup. Introduction Gas/liquid two-phase flow in pipes is a common occurrence in the petroleum, chemical, nuclear, and geothermal industries. In the petroleum industry, it is encountered in the production and transportation of oil and gas. Accurate prediction of the flow pattern, pressure drop, and liquid holdup is imperative for the design of production and transport systems. High-viscosity oils are discovered and produced all around the world. High-viscosity or "heavy oil" has become one of the most important future hydrocarbon resources, with ever-increasing world energy demand and depletion of conventional oils. Almost all flow models have viscosity as an intrinsic variable. Two-phase flows are expected to exhibit significantly different behavior for higher viscosity oils. Many flow behaviors will be affected by the liquid viscosity, including droplet formation, surface waves, bubble entrainment, slug mixing zones, and even three-phase stratified flow. Furthermore, the impact of low-Reynolds-number oil flows in combination with high-Reynolds-number gas and water flows may yield new flow patterns and concomitant pressure-drop behaviors. The literature is awash with two-phase studies addressing mainly the flow behavior for low-viscosity liquids and gases. However, very few studies in the literature have addressed high-viscosity multiphase flow behavior. In this literature review, the state-of-the-art of two-phase flow is first summarized. Then, the studies addressing the effects of liquid viscosity on two-phase oil/gas flow behavior are reviewed.