Cross-linked Langmuir-Blodgett films of isopentylcellulose cinnamate (IPCC) sandwiched between a polymeric diffusion couple provide an ideal system for probing fundamental transport processes across ultrathin (approximate to6 nm) membranes. Neutron reflectivity experiments for polystyrene/IPCC/polystyrene trilayers (with one perdeuterated polystyrene layer) reveal that the isotopically labeled (perdeuterated) species crosses the membrane faster than normal polystyrene of the same size resulting in displacement of the free-standing membrane. Mismatched polystyrene molecular masses confirm isotopic selectivity and also indicate size discrimination by the membrane. The membrane displacement is reminiscent of but different from Kirkendall effect experiments in metallic systems and marker displacement studies in polymers.