A study was undertaken of the air preheater ash deposit in a 140 MW coal-fired traveling-grate boiler. Several ash samples were collected in the selected regions of the air preheater along with samples of the feed coal. The ash samples were examined using combination of ash X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis to determine the chemistry and mineralogy. The ash samples were also examined using scanning electron microscopy and energy dispersive spectrum (SEM-EDS) analysis to determine the microstructure and phase constitution. The analysis results showed that the sulfur content of ash deposits varied with the temperature of the gas flue. The higher amounts of sulfur in ash deposits proved to be sulfates. It appeared that sulfuric acid was a driving factor in the formation of the deposition. The sulfates probably acted as cementing agent, giving the deposit its hardness and thickness.