An experimental study has been made of oil-water core-annular flow in a horizontal pipe with special attention for the influence of the oil viscosity on the pressure drop. For that purpose a heating system has been installed and configured that is able to control the oil temperature, such that the oil viscosity could be varied between 3000 cSt at 20 °C and 400 cSt at 50 °C. The oil flow rate was kept at a constant value of 0.35 l/s, whereas the watercut was varied between 9% and 25%. The measured pressure drop is scaled with the calculated pressure drop of only oil flowing at the same flow rate and viscosity. The main conclusion is that for a large oil viscosity the scaled pressure drop is almost independent of the watercut, whereas with decreasing viscosity the scaled pressure drop becomes strongly dependent on the watercut. Visualisation of the oil-water interface shows a more irregular wave shape with smaller wave lengths when the viscosity is decreased. There is very good agreement between the predictions of the model of Ullmann & Brauner for the scaled pressure drop and the measurements.