Full-scale experiments were conducted in order to investigate flow pattern transitions in horizontal pipelines carrying oil-water mixtures. In the experiments, a 16-in. pipeline conveying light crude oil was used. The line was connected to a freshwater network to control the input water volume fraction. A gate valve installed at the pipeline inlet controlled the oil flow rate. The transition from stratified flow to dispersed flow was determined by measuring the transversal water fraction profile. For this purpose, a special device, the multi-point sampling probe, was designed and installed into the pipeline. The probe has movable sampling tubes that allow taking samples simultaneously at six points along the diameter of the pipe. The rate of withdrawal of each sample was adjusted by a needle valve according to the mixture velocity in order to minimize the effect of the probe on the measured water fraction profile. The samples were analyzed for water content in a laboratory using a standard method for determining the water fraction in crude oils. Based on the data obtained, a flow pattern map was constructed. The experimental stratified/nonstratified transition boundary was compared with two theoretical criteria obtained in the linear stability analysis of stratified two-phase liquid-liquid flow. The results of this study can be useful for the design and operation of pipelines transporting crude oil, as well as for the validation of multifield multidimensional models of two-phase flow. [S0195-0738(00)00404-0].