Mixtures of colloidal particles of essentially two different sizes are commonly used in technological applications such as film formation on surfaces, paper coatings, etc. Subtle behaviour and changes in rheology due to the presence of small particles are reported. This proposal will exploit the both the low Q and wide range of Q tavailable on SANS 2D to investigate ¿high concentration, crystalline¿ phases of polystyrene latex binary mixtures. This data should allow a comparison of hard sphere packing in other studies with the long range order in such charge stabilised systems. The unique contrast variation possibilities available with neutron scattering are essential for this type of study.