The adsorption isotherm of a soluble anionic surfactant was measured and discussed within the framework of established thermodynamical models. The adsorbed amount derived by these models was compared with the results obtained by second harmonic generation, SHG. SHG can be used as a highly surface specific probe which allows the determination of orientation and number density. Since no thermodynamic quantities are involved in the interpretation of SHG measurements, the data can be used to assess the validity and underlying assumptions of thermodynamic adsorption models. The SHG experiments reveal a good agreement with the Frumkin model. No indication of a subsurface layer can be found.