A unique combination of acid treatment, aqueous colloidal processing, and spark-plasma sintering (SPS) has been used to fabricate high-density Al2O3 /single-wall carbon nanotube (SWNT) composites with well-distributed SWNTs and other carbon nanostructures ('nano-onions', diamond) at Al2O3 grain boundaries. This approach could be used to obtain well-controlled microstructures of ceramic/SWNT composites for tailored mechanical, electrical, and thermal properties. In addition, the colloidal approach for dispersing SWNTs presented here could be used for the controlled manipulation of SWNTs.