A tantalum/niobium concentrate has been ball-milled for up to 50 h in a laboratory mill without apparent phase change, other than a decrease in unit cell symmetry and some amorphisation. The extent of dissolution in a mixture of hydrochloric acid and sodium fluoride increased with milling time. For a powder milled for 50 h, over 95% of the concentrate were solubilised in 24 h compared with similar to 50% in 48 h for a 2-h milled powder. The kinetics of dissolution changed with the milling time, indicating a change in the rate-determining step due to milling. These effects were attributed to the decrease in crystallinity of the powder, giving a substantial increase in the lattice strain and in unsatisfied surface bonds where dissolution was thought to occur. (C) 2001 Elsevier Science B.V. All rights reserved.