The interlayer space of fixed-charge, expandable, layered aluminosilicate clays is considered filled with a two-component dielectric solution and treated as an open thermodynamic system. The composition of the interlayer solution, relative to the external solution, is obtained by performing a thermodynamic analysis that includes the work of polarization of the dielectric solution. The theory predicts that compositional changes are most dramatic when the component having greater polarity is present to a smaller extent (less than or equal to 20 mol %). Furthermore, the theory is shown to contribute significantly to the understanding of previously published adsorption isotherm data as well as swelling behavior of organo-clays in organic solvents.