Adsorption of acetonitrile from toluene solution at a liquid-solid interface under pressures of up to 300 MPa was investigated by IR spectroscopy, The CN stretching vibration bands (upsilon(CN)) Of adsorbed acetonitrile were observed at higher frequencies than those of the same species in the liquid phase. The shift on alumina-pillared montmorillonite (ALPM) was the largest (ca. 8 cm(-1)) for the adsorbents studied. The upsilon(CN) intensities of adsorbed acetonitrile on ALPM and on alumina (Al2O3) considerably increased with increased pressure, indicating an increase acetonitrile adsorption due to compression, It was concluded that the total volume of the system was reduced by adsorption, and that the reduction was brought about not only by the formation of an adsorption bond but also by the change in the solvation of the adsorbate in the adsorbent pore.