The surface charge of an anionic or cationic monolayer at the air-water interface is compensated by the oppositely charged counterions that are either located within the diffuse layer or directly bound to the interface. In this paper we demonstrate that ellipsometric measurements allow a direct determination of the prevailing ion distribution. A simple analytical expression is derived, which can be used to retrieve the number of ions within the compact layer. This formalism is applied to the experimental data gained on aqueous alkyl pyridinium bromide solutions. The analysis of the experimental ellipsometric isotherm provides evidence for the existence of an ion condensation just before the cmc. The derived formalism relates ellipsometric signal with the surface charge and should be of utmost importance to discard or proove the recently introduced concept of fluctuation driven ion condensation at a charged surface.