This study explores the complex fluid flow behavior adjacent to the interface between parallel layers of gas and liquid. Using water and nitrogen as working fluids, the interface is examined experimentally using high-speed video, and the flow structure predicted using FLUENT. The computational model is used to analyze the gas flow near the interface by isolating and examining a domain that represents an instantaneous snapshot of the wavy interface. Both the observed and computed interfaces show appreciable interfacial waviness, which increases in intensity with increasing flow rates; they also show gas entrainment effects at high flow rates. The computed results show turbulence is completely suppressed along the interface by surface tension. Computed velocity vector plots, contour plots and flow streamlines show interfacial flow separation on the gas side, and these effects are amplified with increasing gas Reynolds number. This produces form drag along the wavy interface in addition to the viscous drag. The interfacial viscous and form drag components increase monotonically with increasing ratio of wave height to wavelength because of the increased frictional resistance and flow separation effects, respectively. A new relation for the interfacial friction factor is derived from the computational results, which agrees well with prior turbulent flow correlations. (C) 2015 Elsevier Ltd. All rights reserved.