The elasticity of multicomponent closed and partially open thin films is analyzed. The elastic modulus of a completely closed film is shown to be additive with respect to contributions that are due to the Gibbs elasticity and disjoining pressure. In the case of only one closed component (e.g., a surfactant), both contributions are inseparably related to each other and may be expressed one through the other. For films of surfactant solutions, the resultant expressions make it possible to explain the marked maximum (observed in many experiments) of the foaming capacity of solutions and stability of foams in the vicinity of the critical micellization concentration.