Partially deuterated Cd−arachidate Langmuir−Blodgett multilayers were subjected to a gas atmosphere of n-pentane and n-hexane at room temperature. The gas−film interaction was studied by in-situ X-ray and neutron specular scattering through a gas-flow box. Both methods show an increase of the small-angle Bragg peak intensities during the gas treatment while the intensities of Kiessig oscillations decrease at the same time. The change of intensities is partially reversible after purging with air. Neutron scattering reveals that the increase of structural Bragg peaks is also accompanied by a decrease of superstructure peaks originated by the scattering length density difference between the sequentially stacked deuterated and nondeuterated chains. This indicates a gas-assisted intermixing of amphiphilic molecules in the vertical direction. The observed effect is much larger than in the case that the short-chain molecules occupy defect sites of the virgin Langmuir−Blodgett film. It can be explained by assumin...