Over the past few decades, detonation nanodiamonds (NDs) have gained increased attention due to their unique physicochemical properties. Various methods for preparation of ND suspensions have been introduced. This paper presents thermally annealed nanodiamonds dispersed via sonication and separated by centrifugation in deionized water and dimethylformamide in five weight concentrations ranging from 0.05 to 1 wt%. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were applied to study the thermal behavior of NDs. Crystallographic properties of air-annealed and dispersed NDs were examined by means of X-ray diffraction (XRD). Nanodiamond dispersions were analyzed by static light scattering (SLS), dynamic light scattering (DLS), ultra-small- and small-angle X-ray scattering (USAXS/SAXS), and high-resolution transmission electron microscopy (HRTEM). SLS and DLS give similar results of ND− aggregates mean size between ~ 61 and 73 nm, regardless of solvent type and nanoparticle concentration. For dispersions with increasing concentrations of NDs, neither increased aggregate size nor different kinetics of separation during sonication and centrifugation were observed. USAXS/SAXS provided the aggregates size (2Rg) in the range from 57 to 65 nm and size of primary particles from 5.4 to 5.8 nm. HRTEM also showed presence of larger aggregates with tens of nanometers in size in both water and DMF dispersions, and size of primary particles ranging from 5.5 to 6 nm in very good agreement with SAXS. Graphical abstract.