We have investigated the spontaneous self-assembly of solid, mesostructured films that form at the air-solution interface on solutions containing a neutral water-soluble polymer and catanionic surfactant mixtures of hexadecyltrimethylammonium bromide (CTAB) and sodium dodecylsulphate (SDS). The formation processes and structures were probed using neutron reflectivity, X-ray reflectivity, off-specular time-resolved scattering, and grazing incidence diffraction. The mesostructures of films prepared with polyethylene oxide, polyethylenimine, and polyacrylamide at various cationic/anionic surfactant molar ratios are compared. The results suggest that polymers having a weak interaction with the surfactants cause a depletion aggregation process that results in a lamellar phase, whereas polymers having a stronger interaction with the surfactants produce more complex mesostructures in the films.