In the present proposal neutron reflectivity will be used in combination with contrast variation to detect changes in the presence (and size) of microdomains in monolayer films as a function of lipid composition and film compression. In addition, the effects on the formation of rafts by film compression will also be measured. Ternary mixtures of palmitoyloleylphosphatydlcholine (POPC), palmitoylsphingomyelin (PSM) and cholesterol will be used as the phase behaviour of these common eukaryotic plasma membrane constituents is well known. The data obtained will complement Langmuir trough and Brewster Angle microscopic data and provide a more comprehensive knowledge of domain formation.