Purity and pre-compaction of powder precursors are the key factors determining the quality of nanoceramics. We investigated desorption and re-sorption of gaseous impurities on nanocrystalline powders of diamond and silicon carbide using thermo-gravimetry and mass spectroscopy techniques. Compaction was investigated by means of SAXS and gas porosimetry under pressures up to 7.7GPa. The amounts of gases adsorbed by nanocrystalline powders stored in air correspond to one monolayer of molecules. In a flow of purging gas (Ar, He) weakly adsorbed molecules (mainly H₂O) desorb at temperatures below 200^oC. Desorption of relatively strongly bound molecules (O₂, CO, CO₂, N₂) requires temperatures up to 500^oC. Above 500^oC oxidation of the surface by residual oxygen from the purging gas takes place. Freshly desorbed samples exposed to air for 0.5 h not show any considerable contamination. Those results provide a guidance for conditions of cleaning the nanopowders before sintering. Effective densification of nanopowders requires application of extreme pressures. At 2.5GPa the relative density of both diamond and SiC compacts is only about 0.5. At 7.7GPa the densities reach 0.66 for SiC and 0.75 for diamond. Extreme pressures close pores much more efficiently in diamond than in SiC. This difference is due to a difference in the morphology of the two materials. Diamond nanopowders, produced by detonation technique, undergo an intense chemical treatment what causes a considerable agglomeration. Silicon carbide powders (synthesized by gaseous methods) exhibit a pronounced fractal structure. SAXS data show that during compaction this structure is being preserved and the density of the compact remains low until the pressure reaches approx. 1GPa. Above that pressure the voids in the compact form 2-dimensional aggregates. As the pressure increases those aggregates gradually dissociate into one-dimensional chains of voids and, finally (at approx. 6GPa) form isolated voids.