In order to understand the properties of concentrated dispersions it is necessary to obtain the spatial and temporal locations of the particles in the system as a function of both the volume fraction and the electrolyte concentration and to correlate this with theoretical models. A useful experimental technique for examination of the static structure factor is provided by small-angle neutron scattering, using cold neutrons with a wavelength of ca. 10 Å, since the scattering vectors then available are appropriate for systems containing small particles over a wide range of volume fractions. Polystyrene latices consisting of spherical particles of radius 157 Å, with a narrow distribution of particle sizes, provide an excellent system for such studies. Structures are reported for these latices covering a range of volume fractions from 0.01 to 0.14 and electrolyte concentrations from very low values, ion-exchanged systems, up to 5 × 10^–3 mol dm^–3 sodium chloride. The structure factors obtained are compared with various theoretical models for electrostatically interacting systems.