The results of infiltration (wetting) experiments conducted on expansive soils demonstrate several requirements and constraints to the techniques used for the study of wetting performance of such soils. In part, these constraints are a necessary outcome of limitations imposed by the difficulties (impossibility?) of measurement of swelling pressure at the wetting front. To provide a better insight into the development of swelling and reaction pressure in the soil during the wetting process, the energies of interaction between particles and water are examined, especially in regard to those forces developed in the Stern layer. The Grahame modification of the Stern layer has been used in this study to provide the basis for calculations of interaction energies in the inner and outer Helmholtz planes. Comparison with high-pressure consolidation of a sodium montmorillonite at very close particle separation distances suggests that the addition of the energies of interaction developed in the Stern layer to the Gouy–Chapman model would permit the double-layer model to be extended to close particle spacings. Whether this is sufficient to account for the stage I wetting process is a question that remains to be further studied. For the present, the test results suggest that the expression for the total soil–water potential ψ should account for those forces of interaction, thereby providing a better account of the physical processes involved in wetting of the expansive clay and a more realistic diffusion coefficient for the total wetting process. Key words : soil-water potential, osmotic potential, swelling pressure, volume change, wetting front, Stern layer, inner Helmholtz plane, outer Helmholtz plane, Coulombic forces, dipole–dipole interaction, ion–dipole interaction.