Ellipsometry is a well-established, nondestructive optical method for the characterization of thin films. An ellipsometric experiment yields in the thin film limit only a single parameter eta, which is related to changes in the state of polarization caused by reflection. The ellipsometric quantity is only subject to certain conditions proportional to the adsorbed amount Gamma. The necessary requirements leading to the proportionably are not met for adsorption layers of soluble surfactants at the air-water interface since the dielectric constants epsilon of all media are very similar. It is not possible to establish from first principles (Maxwells equations) a unique relation between state of the monolayer and eta. The derived expression cannot be inverted, and it is not justified to assume a linear relation between eta and the surface excess Gamma. The aim of this contribution is to obtain an understanding what eta represents for soluble surfactants at the air-water interface. For the purpose of this study a soluble surfactant was designed which possesses a sufficiently high hyperpolarizability to enable surface second harmonic generation (SHG) in reflection mode to be performed. Polarization dependent SHG measurements were used to determine the orientation, the surface excess, and the symmetry of the interface. These data were used to assess the meaning of ellipsometric measurements. The comparison reveals that the relation between surface coverage and ellipsometric signal is nonlinear. The ellipsometric isotherm increases at low concentration and possesses a maximum at an intermediate coverage and then even decreases with increasing surface excess. These features can be understood in terms of changes in the orientation of the aliphatic tails of the amphiphile and by the prevailing ion distribution at the interface. Ellipsometry is therefore not a suitable alternative to surface tension measurements, neutron reflectometry, or nonlinear optical investigations for the determination of the surface excess of soluble surfactants although it is convenient technique to characterize qualitatively local and temporal variations of the molecular density at fluid interfaces.