Abstract: Although usual pressures have typically a weak effect on the properties of condensed phases and their surface layers, a parameter has been found in the surface physical chemistry—a contact angle at a three-phase contact line—that is rather sensitive to hydrostatic pressure. Experiments with an air bubble adhered to a solid surface immersed in water have shown that an increase in the hydrostatic pressure by less than two times causes a growth of the contact angle by more than 10°, if the angle is markedly smaller than 90°. Therewith, the three-phase contact line remains immobile, and only the liquid−gas interface changes its orientation. If the angle (no matter, acute or obtuse) is close to 90°, the three-phase contact line acquires mobility as an alternative way to reach an equilibrium. A thermodynamic theory has been developed on the basis of the generalized Young equation to explain these phenomena. It has been shown that, when the three-phase contact line is fixed, a growth of the pressure in a liquid always leads to a rise in the contact angle.