This paper is devoted to slip phenomenon between the phases that occurs in unstable two-phase water-oil flow systems in a horizontal pipe. The emphasis is placed on the relation between the slip and the real (in situ) water fraction in a flowing mixture, as well as the substitute physical properties of the whole two-phase system. The experimental data collected throughout research served for the evaluation of the accuracy of the methods of real phase fraction in a water-oil flow system in horizontal pipes as they were referred to in the bibliography. Subsequently we have suggested tile author indicate a method of determination of the fraction for two-phase liquid systems like O/W, W/O and W + O. In order to establish the specific equations, the drift-flux model has been used here. (C) 2008 Elsevier Ltd. All rights reserved.