A number of important biological processes, such as intracellular transport and signal transduction depend on the viscosity and the distribution of microdomains in cells. We have recently suggested a novel method of imaging viscosity in single live cells based on fluorescence detection from small fluorophores called molecular rotors. One of our major goals is to create a rotor suitable for probing viscosity inside a plasma membrane of living cells. So far, we have designed two classes of suitable chromophores: BODIPY rotors and conjugated porphyrin dimers. This proposal will use small-angle neutron scattering (SANS) to determine the precise positioning of 5 membrane-soluble molecular rotors with respect to bi-layer structure, using model membrane systems.