A tantalum/niobium concentrate has been mechanically milled with magnesium and either graphite or nitrogen for 100 h. Directly after milling the formation of MgO was evident in both systems, a mixed metal carbide (Ta, Nb)C was also present in the carbon system. A cubic phase, close to that expected for the mixed metal nitride, was found in the powder milled under nitrogen. Annealing of the powders showed the reaction was incomplete with reduced oxide phases present. The carbide/nitride phases were readily separated from the MgO by acid leaching which left powders <5 mu m in size. The leaching preferentially dissolved the smaller, more highly strained crystallites in both cases.