The study of interactions between a polyelectrolyte (sodium polystyrene sulfonate, PSSt) or its water-soluble monomer (SSt) at different concentrations and a monolayer of dioctadecyldimethylammonium bromide (DODA) has been investigated. The monolayer phase behavior and structure at the air-water interface were studied by surface pressure-area isotherms and grazing incidence X-ray diffraction measurements. DODA molecules organize following a rectangular unit cell in all three subphases (pure water, water containing SSt or PSSt). The presence of polyelectrolytes in the subphase decreases, on one hand, the tilt angle and the mean area per molecule in the condensed phase, revealing a higher 2D density in this state, and, on the other hand, the amount of organized matter. (c) 2006 Elsevier Inc. All rights reserved.