Organization of complex monolayers of pyrene-3-sulfonate (PyS(-)) and dioctadecyldimethylammonium bromide (DOMA) at the air/water interface has been studied by surface pressure and surface potential measurements, by fluorescence,spectroscopy, and by reflection spectroscopy under normal and oblique incidence of light. Complex monolayers have been prepared using the cospreading technique, i.e., by spreading mixed solutions containing the anchor lipid DOMA and PyS(-) in different molar ratios. It is deduced from various techniques that in the densely packed PyS(-)/DOMA complex monolayers, a densely packed submonolayer of PyS(-) is formed under the head groups of the anchor Lipid monolayer. Reflection spectroscopic measurements provide additional information on the molecular arrangements at the interface and show that the pyrene chromophores do not lie parallel to the interface but are inclined. The influence of subphase composition on the monolayer organization and processes at the interface has also been studied. Excimer emission is observed in the mixed monolayers spread on the water subphase. Introduction of negatively charged counterions in the subphase leads to the dominance of monomer emission. The influence of the polysaccharide xanthan was investigated in more detail. The results are discussed in terms of a phenomenological model of molecular organization.