This study aimed to investigate the changes in oil shale (OS) ash as treated by the aerobic combustion process (ACP) and anaerobic pyrolysis process (APP). ACP ashes had all major oxides, and the trace elements content increased with burning temperature increasing up to 1000 degrees C. Meanwhile, APP ashes had a slight enrichment in V, Cr, Ni, Cu and U, but were depleted in As, Zn and Cd. No change in the ashes mineralogy was observed. X-ray absorption near edge structure (XANES) measurements showed that toxic elements converted from lower oxidation states into more mobile higher oxidation species such as Cr6+, AS(5+) and V5+. Moreover, the leaching test on ACP ashes gave alkaline elutes rich in Cr, i.e. 30 mg/L. Based on this result, the expected huge friable tailings produced would interact with rain water which might result in toxic elements-rich leachates, thus causing severe pollution.