The structure of ionic liquid (IL)-gas interfaces is currently a hot topic in the field and understanding of the surface structure of pure ILs is improving. However, very little is known about the surface structure and dynamics of mixtures of ILs. This proposal involves the use of neutron reflectometry measurements to investigate the surface composition and structure of the ionic liquids [EMIM][Tf2N] and [C12MIM][Tf2N] and their binary mixtures. Isotopic contrast variation will be employed in order to probe the surface structure and to investigate the surface activity of the different cations as a function of the bulk composition. These data, alongside SANS measurements and complementary reactive-oxygen-atom scattering experiments, will enable a detailed understanding of nano-segregation and structuring effects (in both the bulk and at the surface) in these IL mixtures.