A theory is presented to describe the increase in the surface pressure of a mixed adsorbed monolayer under linear compression. This mixed monolayer contains a very soluble surfactant and a less soluble one as a minor component. This theory is a simplification of a more general one with two not very soluble surfactants. The presented theory is verified by experiments with SDS and dodecanol. Good agreement between theory and experiment is obtained. For micellar systems, the minor component is solubilized in the micelles and the compression curves are different from those for concentrations below the CMC. From the compression data the elasticity of the monolayer as a function of time is obtained.