Mineral composition of oil shale ash stored in the ash fields of the Balti Power Plant was determined by X-ray diffractometry. Changes in the content of the main compounds and new compounds formed during the storage of ashes under natural conditions were investigated. The composition of the ash collected from the ash fields was substantially different from the composition of the ash discharged from power plants. Different compounds were detected in the various boreholes as well as at different levels of the same borehole. The results obtained could he used to explain and control the processes taking place in ash fields.