An analysis of high viscosity oil/water flow in horizontal and slightly inclined pipe is presented. We propose experimental flow maps - in particular we focus on the core-annular flow pattern boundary pressure drops, and oil hold-up measurements. Experimental data are used to validate some models present in literature. Since hold-up measurement in liquid-liquid systems is particularly challenging and, therefore, only few data are available in literature, they are analyzed and commented in detail. The slip ratio computed by hold-up data is also provided and analyzed. (C) 2011 Elsevier Ltd. All rights reserved.