The adsorption process of a negatively charged polymer (poly (styrenesulfonate)) to a cationic monolayer surface was studied by means of direct surface forces measurement and surface pressure-area isotherms. The expanded state of the monolayer at the air-water interface is considerably reduced in the presence of the polymer in the subphase. Maximum contraction of the surface monolayer is attained at a polymer concentration of ~1 mg/L, or 5 × 10-6 M. The surface forces between the charged surfaces decrease by more than 1 order of magnitude at this polymer concentration. This indicates nearly complete neutralization of the surface by adsorption of poly(styrenesulfonate). This fact and the absence of structural forces in the vicinity of the surface point to flat adsorption of the polymer onto the surface. In contrast, the surface forces reappear at a larger polymer concentration (approximately 5 mg/L), probably due to additional adsorption of the polymer and recharging of the surface.