This study was designed to investigate the nature of nickel and cobalt dissolution from limonite and nontronite ores. Leaching was achieved using 0.2-3 M of citric, lactic and malic acids. The effect of acid type, acid activity, oxygen reduction potential and pulp density on metal dissolution was studied systematically. The effect of leaching on the mineralogy of the ore was investigated by optical microscopy and by synchrotron based X-ray diffraction. From the experiments, it was shown that nickel and cobalt dissolution were dependent on acid activity, oxygen reduction potential, pulp density and mineralogy. The extent of metal dissolution was less affected by the type of acids. The trend of selectivity in limonite and nontronite are Co > Mn > Mg. Metal selectivity was intimately associated with the host minerals containing nickel and cobalt. (C) 2006 Published by Elsevier Ltd.