A novel high-pressure apparatus and technique were developed to measure CO₂/water/solid contact angles in situ for pressures up to 204 bar. For two glass substrates with different hydrophilicities, contact angle increased significantly with CO₂ pressure. As pressure was increased, an increase in the cohesive energy density of CO₂ caused the substrate-CO₂ and water-CO₂ interfacial tensions to decrease, while the water-substrate interfacial tension increased. Contact angle for the more hydrophobic substrate was predicted accurately from the experimental water-CO₂ interfacial tension and an interfacial model which only included long-ranged forces. However, for the more hydrophilic substrate, short-ranged specific interactions due to capping of the silanol groups by physisorbed CO₂ resulted in an unusually large increase in the water-substrate interfacial tension which led to a much larger increase in contact angle than predicted by the model. A novel type of contact angle hysteresis was discovered in which larger contact angle values were observed during depressurization than during pressurization, even down to ambient pressure. Effective receding angles were observed upon pressurization, and effective advancing angles were observed upon depressurization on the basis of movement of the three-phase contact line. The greater degree of hysteresis for the more hydrophilic silica can be attributed in part to the capping of silanol groups with CO₂ . The large effects of CO₂ on the various interfacial energies play a key role in the enhanced ability of CO₂ , relative to many organic solvents, to dry silica surfaces as reported previously on the basis of FTIR spectroscopy (Tripp, C. P.; Combes, J. R. Langmuir 1998, 14, 7348-7352).