AbstractThe structure and interaction of nanodiamond fractal clusters were studied by small-angle X-ray and neutron scattering including contrast variation. The density of nanodiamonds, the aggregation number, and the ratio of aggregated/non-aggregated particles were determined. The analysis of the structure-factor made it possible to obtain the effective potential of cluster–cluster interaction. Branched fractal colloids behave to a certain extent like hard spheres, but the effective correlation length decreases with increasing concentration up to the sol–gel transition. The proposed approach for analyzing cluster–cluster correlations can be used in wider class of liquid systems with nano-sized non-compact inclusions.