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This paper presents a comparative study on ash deposition of two selected coals, Russian coal and lignite, under oxyfuel (O2/CO2) and air combustion conditions. The comparison is based on experimental results and subsequent evaluation of the data and observed trends. Deposited as well as remaining filter ash (fine ash) samples were subjected to XRD and ICP analyses in order to study the chemical composition and mineral transformations undergone in the ash under the combustion conditions. The experimental results show higher deposition propensities under oxyfuel conditions; the possible reasons for this are investigated by analyzing the parameters affecting the ash deposition phenomena. Particle size seems to be larger for the Russian coal oxy-fired ash, leading to increased impaction on the deposition surfaces. The chemical and mineralogical compositions do not seem to differ significantly between air and oxyfuel conditions. The differences in the physical properties of the flue gas between air combustion and oxyfuel combustion, e.g. density, viscosity, molar heat capacity, lead to changes in the flow field (velocities, particle trajectory and temperature) that together with the ash particle size shift seem to play a role in the observed ash deposition phenomena.