An experimental investigation on high-viscosity oil-water flow in a horizontal pipe (I.D. = 26 mm) has been conducted. The oil viscosity investigated varied between 3800 and 16 000 mPa . s. Flow patterns observed in experiments are presented and flow pattern maps are reported. The inversion from oil-continuous to annular-water-continuous is discussed. The average pressure gradient of oil-continuous twophase flow can be reduced before the inversion to annular-water-continuous flow due to partial lubrication from discontinuous water streams. The stable water-lubricated flow develops at a lower input water volume fraction with increase of the superficial oil velocity. An empirical criterion for the formation of stable water-lubricated flow was proposed in terms of the oil phase Froude number and the input water volume fraction. The friction factor of water-lubricated flow can be one to two orders of magnitude higher than that of single-phase water flow and has a faster decrease with increase of the Reynolds number than that of single-phase turbulent flow. These are linked to the oil fouling on the pipe wall. Models for the prediction of the pressure gradient of core flow proposed by different authors in the literature were evaluated with the experimental data.