Poly(styrenesulfonate) (PSS) was absorbed reversibly from aqueous CsCl or NaCl solutions onto oppositely charged DODAB (diocatadecyldimethylammonium bromide) monolayers at the water surface. By adjusting the DODAB lateral density, the surface charge of the liquid interface is varied. The electron density profile of the surface layer was measured with X-ray reflectivity, using counterion contrast. A nat PSS layer is always found, with a thickness corresponding to one to two chain diameters. At low surface charge, the PSS coverage is constant and large, presumably due to hydrophobic attraction. A surface charge overcompensation of up to 6:1 is observed. The PSS coverage increases both with surface charge and with ionic strength. Intriguingly, at high salt (1 M) and low surface charge, beneath the flat concentrated PSS layer a diffuse extended adsorption layer is found. Its thickness decreases on increase bf the surface charge and disappears at ca. 80 Angstrom (2) DODAB molecular area, causing a minimum in PSS coverage. The diffuse extended layer is not found for a short PSS with 52 Angstrom contour length.