The microstructural evolution during solid-state sintering of nanocrystalline WC–10 mass% Co powders, prepared by different ball-milling processes to give rise to diverse mechanical activation states, is investigated. During sintering, two ternary carbides, Co6W6C and Co3W3C, form at the binder phase which surrounds WC particle aggregates. The overall microstructure of the sintered material (including the formation of these carbides) is found to be very sensitive to the initial state of the powders, even more than to the sintering temperature and/or sintering time. In addition, the nanocrystalline nature of the composites is preserved during the solid-state sintering, in part assisted by the formation of these complex carbides and decarburization. This nanostructure yields moderate microhardness and fracture toughness values, in spite of the remaining levels of porosity.