To investigate whether the radiative properties of carbonate rich ash layers in oxyfuel combustion systems might be influenced by the carbonate decomposition to the corresponding oxide, the emittance of Sr, Mg, and Ca carbonates is examined. In addition "synthetic coal ashes" were produced from mixtures of CaCO3 and SiO2 as well as Fe2O3 and SiO2. The mixture ratios of the minerals were varied. All samples were prepared from powders with known particle size fractions of x < 32 mu m and 125 < x < 160 mu m. The powders were investigated for their temperature-dependent normal emittance in a radiation test rig by FT-IR spectroscopy in the temperature range from 500 to 1000 degrees C. The results reveal that the phase transformation from the carbonate to the corresponding oxide has a significant influence on spectral emittance. Whereas the carbonates show characteristic peaks in spectral emittance around 4 mu m which stem from the infrared active CO3 group, these peaks vanish after transformation to the oxide. For CaCO3, the most prominent carbonate in typical coal ashes, the emittance of the oxide is significantly lower than for the carbonate. Such a behavior in terms of total and spectral emittance has also been detected, for example, examining a Ca rich Rhenish lignite. Emittance increases with particle size for all samples. An enrichment of SiO2 with Fe2O3 leads to an increase in emittance.