The flow characteristics of the two-phase system —white mineral oil and water—were examined in a horizontal, smooth, one-inch pipe. Flow conditions were investigated over a range of input oil-water volume ratios from 0.1 to 10 at thirteen superficial water velocities ranging from 0.116 ft./sec. to 3.55 ft./sec. A theoretical analysis of the laminar flow of two immiscible liquids between wide parallel plates yielded a modified parallel plate friction factor based on the water properties and the superficial water velocity. It was evaluated for a number of input oil-water volume ratios and plotted against the superficial water velocity. The experimental pressure drop data that were obtained were correlated using a modified Fanning friction factor which was evaluated for the range of input ratios studied and correlated with the superfical water velocity. A flow pattern correlation was obtained for visually observed types of flow—bubble, stratified and mixed—and it was shown that these patterns occurred in laminar, transitional or turbulent conditions of flow. The theoretical analysis for flow between wide parallel plates was adapted to obtain hold-up relationships, and a plot of the hold-up ratio HR (the input divided by the in situ oil-water volume ratio) versus the input oil-water ratio was constructed. This plot indicated that in the laminar region of flow the hold-up was not dependent on the superficial water velocity but was only a function of liquid viscosity and input ratio. Experimental results for flow in the pipe conformed with this prediction while indicating that in the turbulent region superficial water velocity was also a factor.