Solid-fluid interactions in unsaturated expansive clays can be divided into capillarity and adsorption effects based on their physical mechanisms. Most constitutive models for unsaturated soils are proposed on the basis of the capillarity mechanism, ignoring the contributions of the adsorption effect to mechanical and hydraulic behaviors. For expansive clays, however, the adsorption effect which leads to more complex behavioral characteristics than that in low plasticity clays cannot be ignored. In the light of this, a new binary-medium model for unsaturated expansive clays is proposed, involving a consideration of the solid-fluid interactions stemming from the capillary and the adsorption mechanisms at the same time.Firstly, we assume that expansive clay is a mixture of two ideal parts, i.e. the ideal capillarity part and the ideal adsorption part, and then an ideal capillarity model and an ideal adsorption model, each of which is available for the corresponding ideal part, are established. Furthermore, a participation function is used to reflect the degrees of capillarity effect and adsorption effect. Finally, predictions are performed on the results of the consolidation tests and the cyclical controlled-suction tests published in literature.After comparing predicted results with test results, it is illustrated that the established model can quantitatively predict mechanical and hydraulic behaviors in expansive clays. Most-constitutive models for unsaturated soils are proposed on the basis of the capillarity mechanism, ignoring the contributions of the adsorption effect to mechanical and hydraulic behaviors For expansive clays, however, the adsorption effect which leads to more complex behavioral characteristics than that in low plasticity clays can not be ignored. In the light of this, a new binary-medium model for unsaturated expansive clays is proposed, involving a consideration of the solid-fluid interactions stemming from the capillary and the adsorption mechanisms at the same time. Firstly, we assume that expansive clay is a mixture of two ideal parts, ie the ideal capillarity part and the ideal adsorption part, and then an ideal capillarity model and an ideal adsorption model, each of which is available for the corresponding ideal part, are establ is, a participation function is used to reflect the degrees of capillarity effect and adsorption effect. Finally, predictions are performed on the results of the consolidation tests and the cyclical controlled-suction test published in literature. , it is illustrated that the established model can quantitatively predict mechanical and hydraulic behaviors in expansive clays.