A model is proposed to describe the molar volume changes of adsorbed materials on solid/liquid or liquid/liquid interfaces. The model divides the volume of a two-phase system into three subvolumes : two of them are for the homogeneous phases with volumes calculated from the partial molar volumes of the components in the phases and the third volume arises as the difference between the volume of the real system and the joint volume of the calculated phase volumes. It is shown that this third volume can be further separated into two parts : one of them is always present and resulting from the interaction of the two phases and the other originates from the inhomogeneous distribution of an adsorbing component in the interfacial range of the two phases. High-precision liquid density measurements have also been performed on systems containing sodium dodecyl sulfate (SDDS) dissolved both in water and in polystyrene dispersions with a water medium. From these measurements the apparent molar volume of the SDDS was calculated both in water and in a mixed solvent which in this special case is the dispergated particles with the water medium. This adsorption model is used for the interpretation of the data on the basis of which the apparent molar volume of SDDS in the adsorbed state on the polystyrene/water interface has been obtained as a function of the equilibrium concentration of SDDS in the bulk water. The resulted apparent molar volume function of SDDS in the adsorbed state is interpretable by different types of hydrophobic interactions.