Abstract Non-uniform liquid-gas stratified flow (i.e. flow with an interfacial level gradient, ILG) can exist when high viscosity liquids and/or large diameter tubes are used. The available data, covering a wide range of liquid viscosities (0.8 to 310 mPa·s) and tube diameters (0.0254 m to 0.216 m), are analyzed. It is shown that the Lockhart-Martinelli parameter, χ ≤ 1 is a criterion to determine uniform stratified flow and disappearance of ILG. In addition, interfacial shear stress relationships are also shown to be indicative of different types of stratified flow.