Pressure gradients were measured during the cocurrent flow of a low viscosity oil (1.6 mPa s viscosity and 801 kg/m(3) density) and water in two 1-inch nominal bore horizontal test sections made from stainless steel and acrylic resin, respectively. Measurements were made for mixture velocities from 0.3 to 3.9 m/s and for water volume fractions from 0 to 100%. The main finding is the large difference between the results for the respective tube materials which cannot be explained only in terms of the difference in tube roughness. It is postulated that the different wettability characteristics of the two pipe materials are also responsible for this disparity. Furthermore, it was found that at high mixture velocities, where dispersed flow patterns prevail, there is a peak in pressure gradient during phase inversion and an apparent drag reduction effect when oil is the continuous phase.