The effect of complexation by a polyanion on the structure of air-water monolayers of 4-[4-(didecylamino)styryl]-N-methylpyridinium iodide (10ASP) and 4-[4-(dihexadecylamino)styryl]-N-methylpyridinium iodide (16ASP) was studied by recording reflection spectra as a function of compression. In the case of water subphases, 16ASP displayed a coexistence of monomer and aggregate at all surface pressures while 10ASP showed no aggregation. For both 10ASP and 16ASP on dextran sulfate solution subphases, only monomer was observed. In addition to preventing the formation of aggregates, complexation by the polyanion altered the orientation of the long axis of the chromophore to closer to the surface normal, as evidenced by changes in the absorption maximum.