Abstract: Despite the fact that ordinary pressures typically slightly affect the properties of condensed phases and their surface layers, quite a pressure-sensitive quantity was found in the physical chemistry of surfaces. This is contact angle. In experiments with an air bubble stuck to a solid surface in water, a less than double increase in the hydrostatic pressure led to a more than 10° increase in the contact angle, provided that the contact angle was sufficiently less than 90°. During these changes, the three-phase contact line remained immobile, and the process reduced only to a change in the orientation of the liquid–gas interface. If the angle (be it acute or obtuse) was close to 90°, the three-phase contact line became mobile as an alternative way to reach equilibrium. To explain these phenomena, a thermodynamic theory was formulated. It was shown that, if the three-phase contact line is immobile, an increase in the pressure in the liquid always causes an increase in the contact angle.