Composition, structure, texture and gangue minerals associated with three naturally occurring uraninite samples (USG - Saxony, Germany; UPA - Palette mine, Australia and; UEA - El Sharana mine Australia) were investigated. The UEA sample contained uraninite associated with uranyl oxide species, Pb-rich uranylphosphate and Pb/Al phosphate phase with trace levels of galena and sulphides. The UPA sample contained uraninite associated with meta-autunite, Pb/Al phosphate phase and other minor phases including carbonates, quartz, fluorapatite, gold and various Zn/Fe/Cu sulphides. The USG mineralogy was dominated by the sulphides sphalerite and galena with uraninite only a minor component along with unidentified Fe/Mg-rich aluminosilicate, muscovite, coffinite and other sulphides. All samples exhibited complex textural and chemical modifications compared to stoichiometric UO2. The uraninites were substituted mainly by Ca and radiogenic Pb and were highly altered. Alteration was associated with significant variations in chemistry with unaltered uraninite in samples UPA and UEA typically being richer in U, Ca and Pb compared to altered regions. The altered uraninite also was more hydrated. Altered uraninite in sample USG had higher Fe, Pb, S and As compared to less altered uraninite and was significantly depleted in U and Ca. The high Pb in the uraninite was opposite to that observed in the UEA and UPA samples but may be related to the high abundance of galena in the sample. The compositional, textural and microstructural aspects of uraninite are likely to play an important role in any process to extract uranium from uraninite-containing ores. (C) 2013 Elsevier Ltd. All rights reserved.