The interaction of Mn2+ cations in the subphase with a behenic acid monolayer has been investigated as a function of surface pressure by using both grazing incidence X-ray fluorescence and X-ray reflectivity. The fluorescence probe provides an accurate determination of the number of cations attracted near the surface (0,47 ± 0.06 per amphiphilic molecule), and the reflectivity experiments show that these cations are condensed within an extremely thin region (50.5 nm). The complete structural parameters of the film and the surface roughness are compared to experimental data obtained with the same compound on pure water.