The stratified oil–water flow pattern is common in the petroleum industry, especially in offshore directional wells and pipelines. Previous studies have shown that the phenomenon of flow pattern transition in stratified flow can be related to the interfacial wave structure (problem of hydrodynamic instability). The study of the wavy stratified flow pattern requires the characterization of the interfacial wave properties, i.e., average shape, celerity and geometric properties (amplitude and wavelength) as a function of holdup, inclination angle and phases’ relative velocity. However, the data available in the literature on wavy stratified flow is scanty, especially in inclined pipes and when oil is viscous. This paper presents new geometric and kinematic interfacial wave properties as a function of a proposed two-phase Froude number in the wavy-stratified liquid–liquid flow. The experimental work was conducted in a glass test line of 12 m and 0.026 m i.d., oil (density and viscosity of 828 kg/m3 and 0.3 Pa s at 20 C, respectively) and water as the working fluids at several inclinations from horizontal ( 20 , 10 , 0 , 10 , 20 ). The results suggest a physical relation between wave shape and the hydrodynamic stability of the stratified liquid–liquid flow pattern.