In recent years, it has become understood that rather than being homogenous mixtures, biomembranes show a complex phase behaviour, with regions of uniform phases, and others where the membranes separate into domains. Three component membranes of Sphingomyelin/(D)POPC/Cholesterol have been much studied in recent years as models of eukaryotic membranes. The lateral organisation of these membranes as a function of temperature and composition have been well studied, but little is known about the changes in membrane thickness and rougness between the phases. This information gives insights into changes in lipid packing and membrane fluidity, and so in this experiment we wish to examine the transbilayer structure of the bilayers accross the main thermal 'melting' transition. This will give complementary information to our studies of lateral organisation carried out by BAM.