Equations have been derived to determine the isothermal changes in the intensive and extensive parameters of closed heterogeneous systems during the course of surfactant adsorption. Two cases are examined: 1) a drop of surfactant solution surrounded by vapor in equilibrium with the solution, with a foreign gas present; and 2) a gas bubble in a surfactant solution. The regions of stability of these systems have been established. The volume stability of fluid spherical objects in closed systems may be affected substantially by not only the Marangoni elasticity of the surface layer, but also by the compressibility of the liquid in the case of bubbles and the osmotic pressure of the surfactant in the case of drops, with the osmotic pressure in the drop playing the same role in its stability as the pressure of foreign gas in a bubble.