The water/air interface of dilute mixed solutions of dodecyl trimethylammonium bromide and different non-surface-active anionic polyelectrolytes has been investigated by measuring the surface tensions and the X-ray reflectivities. A strong synergistic lowering of the surface tension is found for all surfactant/polyelectrolyte mixtures. This decrease is caused by the formation of polyelectrolyte/surfactane complexes at the surface. It has been detected that these complexes form a relatively dense surface layer. When the surfactant and/or polyelectrolyte concentration is varied, the thickness and density of these surface layers remain nearly constant for a given polyelectrolyte. However, although all the different polyelectrolytes adsorb in a flat configuration, the properties of the adsorbed layers depend slightly on the nature of the polyelectrolyte.