Oxyfuel ashes are supposed to form more sulfates than ashes from air-fired systems. This can be caused by the increased SO2 concentrations due to intensive flue gas recirculation in oxyfuel systems. Therefore, we investigated the spectral emittance characteristics of typical mineral sulfates in coal ashes, namely Mg and Ca sulfates. The samples were prepared in powder form. Two particle size fractions were examined (x < 32 mu m and 125 < x < 160 mu m). The powders were investigated concerning their temperature-dependent normal emittance in a radiation test rig. Spectral measurements by a Fourier transform infrared spectrometer in the temperature range from 500 to 1000 degrees C were carried out. The results reveal that Ca and Mg sulfates show characteristic SO absorption bands in the wavelength regions from 3 to 4 mu m, from 4.5 to 6 mu m, and from 8 to 9.5 mu m. MgSO4 transforms to MgO at around 930 degrees C. The total emittance of the oxide is significantly reduced by Delta epsilon = 0.15 compared to the sulfate. The small size fractions MgSO4 and CaSO4 undergo sintering when being heated, which influences emittance. An increase of total emittance up to a value of Delta epsilon = 0.08 is detected for CaSO4. Finally, it is shown that emittance increases with particle size (Delta epsilon in total emittance approximately = 0.1).