Two-component stratified flow in a horizontal rectangular duct has been investigated experimentally. The ‘working fluids were a transparent white mineral oil and water, the viscosity ratio being approxima tely 30. The tests encompassed a variety of flow regimes including laminar-laminar, laminar-turbulent, and turbulent-turbulent. In addition, a variety of conditions at the interface between the component flows was encountered and investigated. The experimental results for flow and pressure drop are found to be quite insensitive to the sliape of the interface and even, to the presence of very small waves, when both flow components are laminar. This finding extends the range of validity of flat-interface laminar flow theory. The conditions for the existence of various flow regimes are quantitatively characterized. It was demonstrated that interfacial waves could occur even when both component flows were laminar, and that laminar and turbulent flows could coexist in the separate layers. Results from depth-probe surveys and- photographic visualization of the flow are also presented.