Many biological processes occur at the interface of cells, at the so called cell membrane, such as internalization of biological nanoparticles, including virions, exosomes and nanoparticle-based drug-delivery vehicles. In this project, we plan to use neutron reflectometry (NR) to measure the distances between a supported cell-membrane mimic and different models of lipid-based biological nanoparticles. In this way, we will assess how the distance between these entities is influenced by external components, e.g. charge and ligand-receptor density, to thereby gain new knowledge on how e.g. binding-induced nanoparticle deformation and membrane fusion are controlled by inter-membrane distance. This project should be the first of its kind to address such questions using NR, and will contribute improved determination of inter-membrane distances in biologically important processes.