Theories of osmosis in groundwater flow are increasingly used to explain anomalies of salinity in clayey environments. However, predictive modelling through mathematical analysis can hardly be found in literature. In this paper, a model of chemical osmosis based on non-equilibrium thermodynamics, is used to predict the evolution of pressure and salinity in a clay membrane. Analysis of this model reveals simplifications that hold for specific situations. Two experiments from literature serve to show that the analytical modelling solution agrees with numerical and experimental results. Moreover, it is shown that the commonly applied Boussinesq approximation necessarily does not hold when osmosis is involved. Indeed, the clay system must be able to store the excess flow of water induced by osmosis. © 2004 Elsevier Ltd. All rights reserved.