The main objective of the present study is to gain an understanding of the nature of the interaction of ¿phospholipid membranes¿ with small, interfering RNA (siRNA) in the presence of divalent calcium ions. This objective will be achieved using a combination of neutron specular reflectivity (NSR) measurements on a floating supported phospholipid bilayer (with and without the presence of a neutral helper lipid) together with contrast variation to highlight the various components of the systems under study. The detailed structural information resulting from these studies will be used to aid the rationale design of improved gene delivery vectors. Neutron reflectivity measurements using contrast variation is the only technique available to yield the required level of structural information.