In this paper, the effect of oxygen uptake on the structure and properties of lignites was investigated using five different Turkish lignites. Lignite samples were oxidised in aqueous medium at 423 K under 1.5 MPa partial pressure of oxygen for 60 min. Relations between the oxygen uptake and the elemental compositions of the lignite samples were investigated. FT-i.r. technique was applied to the original and oxidised lignite samples to investigate functional group analyses. Mercury porosimetry analyses of the samples were carried out up to about 220 MPa. Effect of oxygen uptake on the bulk density, apparent density, porosity, and pore radius of the lignite samples wen compared. Differential Thermogravimetry (DTG) technique was performed to compare the thermal reactivity of the original and oxidised lignite samples. (C) 2000 Elsevier Science Ltd. All rights reserved.