There is considerable effort being put into the modelling of the outer layer of skin, the stratum corneum, because of impending EU restrictions on the use of animal skin for cosmetic testing. Although the stratum corneum is a multilayer structure there is thought to be significant lateral inhomogeneity in the individual layers. The evidence mainly comes from deposited monolayers of mixtures of the main components, fatty acid, cholesterol and sphingolipid, and suggests phase separation on the nanoscale (< 100nm), but the results may be affected by the transfer to the solid substrate. We propose to use neutron reflectrometry to follow changes in the structure of the monolayer on water using perdeuterated fatty acid and partially deuterated sphingosine to obtain contrast with water and within the layer. We will look for in-plane scattering and changes in the surface normal structure.