The composition of oil shale ashes from pulverized firing (PF) and circulating fluidized-bed boiler combustion (CFB) systems at the Eesti and the Balti Thermal Power Plants was studied by means of quantitative X-ray diffraction, chemical analysis and scanning electron microscopy. The composition of ash varies systematically in the boiler gas pass, depending on the firing technology. PF ashes are dominated by free lime, whose content decreases from the furnace to the last fields of electrostatic precipitators. Same trends can be observed within residual fraction (i.e. mineral phases remaining in firing process) and Ca-silicates. Anhydrite, on the other hand, concentrates into the end of the gas pass. CFB ash shows more complex variations, and on several occasion the changes in certain phases are opposite to variations in respective PF ashes. Comparison of the phase composition of ashes from the same type of boilers in the Eesti and Balti TPP show remarkable differences in Ca-silicate phases. To our best knowledge, earlier mineralogical studies have not addressed compositional variations of ashes from two separate power plants. These differences are probably related to the heterogeneity of oil shale fuel, not so much to combustion temperature.