The true surface tension value of a solvent or a solution, being an intensive thermodynamic quantity, should not depend on extensive parameters such as the diameter of a container or the filling height of the liquid. However, here we describe how a so far unknown phenomenon, that of drastically decreasing the surface tension of surfactant solutions by lowering the solution's level in nonwetting containers, can be achieved. This effect can be explained by removal of the adsorbed surfactant material from the solid/liquid interface and spreading at the air/liquid interface. The surfactant adsorption at the solid/solution interface can be estimated from the accompanying changes of the solution's surface tension. It depends on the structure, concentration, and purity of the surfactant as well as on the properties of the nonwetting surface. Generally, surfactant adsorption at the solid/liquid interface is smaller than that at the air/liquid interface. The procedure described allows compression of adsorption layers of soluble surfactants at the air/water interface without applying a movable barrier.