Deposition of ash on heat transfer surfaces is a major factor in determining boiler performance. Deposits so formed consist of tightly bound ``inside" deposits close to the heat transfer surface and loosely bound ``outer" deposits that are easily dislodged. This paper describes research focusing on both the composition of inner and outer deposits, and also on the rate of deposition of the inner deposit layer. Presented are selected results from a wide ranging study concerned with mechanisms governing the formation of fouling deposits during air and oxy-coal combustion in a 100 kW test rig. The primary focus is on establishing a relationship between the size segregated composition of the coal ash aerosol, and the spatially resolved composition of the deposit layer, and this is accomplished by experimentation that addresses the impacts of conversion from air-firing to oxy-firing. Experiments involved three different coals ( Powder River Basin or PRB, Illinois and a 60/40 Blend of Illinois and PRB), each burned under air-and oxy-coal combustion conditions. Data for the PRB coal suggested that changes in the composition of the inner deposit layer were caused by variations in sub-micron aerosol compositions, but that these occurred only when there were significant changes in flame temperature. An increase of the latter also led to increasing submicron particle concentrations and deposition rates. Although both aerosol and deposit compositions depended greatly on coal compositions, rates of deposition of the inner deposit layer correlate linearly with the concentration of the submicron aerosol in the flue gas, for all three coals, and under all air- and oxy-firing conditions for which data are available. This result suggests the amount rather than the ( variable) composition of the submicron aerosol is important. There was no correlation between deposition rates and total ash concentrations. (C) 2016 Elsevier Ltd. All rights reserved.