Small-angle neutron scattering (SANS) and light scattering have been used to study the interactions between particles (diameter < 20 nm) in silica and ceria sols covering a wide range of concentration. Structure factors, S(Q), have been determined which have been compared with those calculated for a model hard-sphere (HS) potential. At low sol concentrations ( 10–2 g cm–3) interactions are dominated by double-layer repulsion which is most pronounced at low electrolyte concentrations (ca. 10–4 mol dm–3). At higher sol concentrations ( 10–1 g cm–3) the effects of electrolyte concentration become less important and the form of S(Q) is similar to that of a HS system, in which the effective interaction diameter is greater than the diameter of the sol particles. Preliminary SANS studies on mixtures of titania and iron oxide sols using the contrast variation technique are also described from which information on the compatibility and structure of each component has been obtained.