Recent experiments on SANS2D have indicated that small colloidal particles can act as macroions that screen the charged interactions of larger particles. This can cause changes in the crystalline order displayed by the large particles and would eventually lead to a region of instability. Undeerstanding this process is important as it can be crucial as regards stability of polydisperse, electrostatically stabilised dispersions. The present proposal is to extend previous studies to high concentrations (up to 25%) and a broader range of size ratios. These experiments will use contrast variation with deuterated and hydrogenous polystyrene latices as model particles. T