Liquid-liquid flow literature proposes models developed to predict quantities and phenomena of interest. once given fluid properties and the features of the flow domain. The validity of any model should be verified through experimental observations, being this practice an effective way to evaluate the model conditions of applicability and possible limitations. Despite the fact that several works have already been proposed on the validation of theoretical models, most of them concern liquids characterised by low viscosity ratio (mu) over bar, while in industrial realities (such as petroleum or food ones) the liquids involved are often characterised by high viscosity ratios. The extension of low-(mu) over bar results to high-p flows is not straightforward, so that it is necessary to validate the models for the latter case specifically. This work presents experimental pressure drops and flow-pattern maps associated to the flow of oil and water in horizontal and slightly inclined pipe, where the chosen liquids are characterised by an oil-to-water viscosity ratio of about 800:1 at 20 degrees C. Various theoretical models have been considered, with particular attention to core-annular flow two-fluid model and oil-in-water dispersion homogeneous no-slip model for the prediction of associated pressure drops, and flow-pattern map transition criteria involving the regimes encountered in the experimental tests. The theoretical predictions have been then compared to the experimental results. A satisfactory agreement has been found especially as concerns pressure drop comparisons. As regards the predicted transition boundaries superimposed on the corresponding flow-pattern maps, the 'free' parameters have been fitted on the basis of experimental results and observations, and the final agreement is good in the prediction of both the core-annular flow region of existence and the transition to oil-in-water dispersion. No conclusion can be expressed on transition criteria involving stratified flow, which only seldom has been observed in the performed experiments. (C) 2008 Elsevier Ltd. All rights reserved.