A novel high-pressure apparatus and technique were developed to measure CO2/water/solid contact angles (theta) in situ for pressures up to 204 bar. For two glass substrates with different hydrophilicities, theta increased significantly with CO2 pressure. As the pressure was increased, an increase in the cohesive energy density of CO2 caused the substrate/CO2 and water/CO2 interfacial tensions (gamma) to decrease, whereas the water/substrate gamma value increased. theta for the more hydrophobic substrate was predicted accurately from the experimental water/CO2 gamma value and an interfacial model that included only long-range forces. However, for the more hydrophilic substrate, short-range specific interactions due to capping of the silanol groups by physisorbed CO2 resulted in an unusually large increase in the water/substrate gamma value, which led to a much larger increase in theta than predicted by the model. A novel type of theta hysteresis was discovered in which larger theta 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).