Experiments were conducted to evaluate the membrane character of native shale samples by measuring the electrochemical potential across the shale membrane. Results suggest that the composition of the interstitial pore fluid in the shale plays a determining role on the establishment of the electrochemical potential and that, in some cases, the behavior of the shales is close to the expected behavior of a perfect cation-selective membrane. Shales with intermediate electrochemical potentials appear to be more sensitive to water-based fluids than shales that are closer to perfect membranes (high electrochemical potential). A model to simulate the transport of water and ions through shales was developed. Hydraulic pressure, concentration, and electric potential gradients are the driving forces for the flow of water and solute between two solutions separated by the shale. The reflection coefficient and the modified diffusion potential are calculated from the model and are used to characterize the membrane behavior of shales. A sensitivity study on the various model parameters was conducted. Results show that the membrane efficiency of the shale is strongly affected by the concentration of the interstitial fluid, the separation distance between the platelets and the type of ions in the membrane. The higher the ion concentration and the interplatelet spacing, the lower the efficiency. Results obtained using the model produce reflection coefficients that are consistent with experimental data. The model provides excellent insight into the physical mechanisms responsible for the membrane behavior of shales and a way to conduct sensitivity studies on the important model parameters. The membrane efficiencies obtained from the model may be used in wellbore stability simulators to account for chemical interactions between shales and water-based drilling fluids.