The effect of varying standard leach parameters (particle size, total iron content, acid addition, oxidation-reduction potential and temperature) on the dissolution of uranium from natural uraninite was studied. Tests were conducted using two uraninite ores from Australia and results contrasted with equivalent studies conducted on a synthetic UO2 sample. Mineralogical studies, including EPMA analysis of selected leach residues, were also carried out. Results indicated that under standard leach conditions (0.15 M H2SO4, 50 degrees C, ORP 460 mV, 6.7E10(-3) M Fe-TOT), both natural samples exhibited lower uranium extraction rates when compared to synthetic UO2. Decreasing particle size, increasing temperature and increasing Fe-TOT all resulted in an increase in the rate of uranium dissolution whereas changing acid concentration and changing the ORP had a negligible effect on dissolution. Minor differences were observed when comparing results between the two natural samples however in all cases the synthetic UO2 proved easier to extract uranium. The changes in dissolution rates between the natural ores and the synthetic uraninite was attributed to lead impurities within the natural uraninites being preferentially leached and then re-precipitated as a sulphate. The precipitated Pb sulphate was observed coating individual uraninite particles and filling pores and fractures, thus preventing further ingress of the leach solution. This study emphasises the uncertainties in extrapolating experimental results obtained using synthetic systems to studies involving real ores. (C) 2013 Elsevier Ltd. All rights reserved.