The efficiency of uranium leaching determines the economic viability of treating low grade uranium deposits, and is quite sensitive to ore characteristics. The interrelationship between mineralogy, mineral liberation and the leaching behaviour of uranium is not well defined. Uraninite's leaching kinetics are well studied, but relatively little leaching research has been conducted for other uranium minerals. Dissolutions higher than 90% are very difficult to achieve under the normal operating conditions employed for acid leaching of South African ores. In this paper, a mineralogy-leachability explanation is presented to rationalise the difficulty in exceeding 90% dissolution from low grade uranium ores in the Vaal River region (averaging 0.3 kg U3O8/t). Based on the findings, further discussed in the paper, it appears that brannerite's intrinsic inertness is responsible for not obtaining optimum recoveries. It is also shown that for low grade uranium ores in the Witwatersrand context, the real value of uranium leaching could lie in the unlocking of extra gold (0.4-0.6 g/t) by the uranium leaching process that typically precedes cyanide leaching of gold. (c) 2007 Elsevier Ltd. All rights reserved.