AbstractThe horizontal flow of equal density oil‐water mixtures was investigated in a 1‐inch diameter laboratory pipeline. Oils of viscosities 6.29, 16.8 and 65.0 centipoise were used in the experiments. Flow patterns, holdup ratios and pressure gradients were investigated for a range of superficial oil velocity from 0.05 to 3.0 ft./sec. and a range of superficial water velocity from 0.1 to 3.5 ft./sec, with input oil‐water ratios ranging from 0.1 to 10.0.Similar series of flow patterns were observed for each oil and were found to be largely independent of the oil viscosity. At high oil‐water ratios oil formed the continuous phase and a water‐drops‐in‐oil regime was noted. As the oil‐water ratio was decreased the flow patterns concentric oil‐in‐water, oil‐slugs‐in‐water, oil‐bubbles‐in‐water and oil‐drops‐in‐water in which water was the continuous phase were observed. The most viscous oil did, however, exhibit anomalous behavior at low superficial water velocities and this is attributed to different interfacial properties.The pressure drops measured indicated that for a given oil flow rate the pressure gradient was reduced to a minimum by the addition of water provided that the oil was not in turbulent flow. Maximum pressure gradient reduction factors varied from 1.7 for the 6.29 centipoise viscosity oil to 10 for the 65.0 centipoise viscosity oil at input oil‐w