In the present proposal small angle neutron scattering will be used in combination with contrast variation as a non-invasive method to detect the presence and size formation of lipid microdomains in vesicles as a function of lipid composition, temperature and vesicle size. The domain forming lipid mixture system to be used in the present study will be a ternary lipid system composed of palmitoyl sphingomyelin / palmitoyl oleyl phosphatdylcholine / cholesterol as the phase behaviour of this mixture is well established. This ternary mixture is a particularly good model for lipid microdomains as the plasma membrane is rich in cholesterol and sphingolipids as well as phospholipid.