The paper deals with simulation of conditions affecting the correctness of the measurement of a solution/membrane equilibrium which is determined by the method based on saturation of the membrane with the solution and subsequent extraction of a component into water or a suitable solution. Attention has been paid to the step when the solution remaining at the membrane surface is removed by washing with water. In this step, an amount of the component can be transferred into rinsing water which represents a loss of the component in the membrane, and the equilibrium is measured incorrectly. In the numerical simulation, the membrane has been considered as a slab, and unsteady-state mass transfer from the membrane into water has been solved. The simulation, whose results are given graphically, has shown that the loss of the component into water increases with increasing washing time, diffusivity of the component in the membrane, and liquid mass transfer coefficient. On the contrary, the loss of the component decreases with increasing partition coefficient and the membrane thickness. Furthermore, it has been found that the relative loss of the component is independent of the initial concentration of the component in the membrane.