The spontaneous formation of multilayer structures at interfaces from surfactant self-assembly is now emerging as an important structural signature associated with a wide range of technological applications and biological function. Neutron reflectivity, NR, has proved to be an ideal probe of such interfacial structures, and is largely responsible for the raised awareness of their existence and potential. This proposal is part of a major programme (supported by the EPSRC, EP/GR065705, and Unilever) to characterise such structures and to determine the major factors with control their formation, in surfactant mixtures and in polymer / surfactant mixtures. We request here beam time on INTER/SURF to extend our study of the formation of multilayer structures at the air-solution interface in surfactant and mixed surfactant systems, induced by the addition of multivalent counterions.