The flow patterns and pressure gradient of immiscible liquids are still subject of immense research interest. This is partly because fluids with different properties exhibit different flow behaviours in different pipe's configurations under different operating conditions. In this study, a combination of oil-water properties (sigma = 20.1 mN/m) not previously reported was used in a 25.4 mm acrylic pipe. Experimental data of flow patterns, pressure gradient and phase inversion in horizontal oil-water flow are presented and analyzed together with comprehensive comments. The effect of oil viscosity on flow structure was assessed by comparing the present work data with those of Angeli and Hewitt (2000) and Raj et al. (2005). The comparison revealed several important findings. For example, the water velocity required to initiate the transition to non-stratified flow at low oil velocities increased as the oil viscosity increased while it decreased at higher oil velocities. The formation of bubbly and annular flows and the extent of dual continuous region were found to increase as the oil-water viscosity ratio increased. Dispersed oil in water appeared earlier when oil viscosity decreased. The effect of oil viscosity on pressure gradient was also investigated by comparing the results with Angeli and Hewitt (1998) and Chakrabarti et al. (2005). One of the main findings is the large difference between the pressure gradient results which is attributed to the difference in oil viscosity. The differences between the results become bigger at higher oil velocities. The largest difference in pressure values was observed in flow region where oil is the continuous phase. On the contrary, for dispersed oil in water (Do/w), the pressure gradient values observed at the same conditions are approximately the same. A simple correlation was developed to predict the pressure gradient in this regime. The correlation was validated using new experimental data. Finally, the effect of oil viscosity on pressure gradient prediction was investigated using the two flow model for stratified flow and the homogenous model for oil dispersed in water. Both models showed better prediction for the low oil viscosities. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.