A study was undertaken of the ash deposits on different positions in a 140 MW coal-fired traveling-grate boiler with ammonia present in the flue gas. Several ash samples were collected from the slag tubes and three-stage heat exchanger of this boiler. The ash samples were examined using the combination of X-ray fluorescence and X-ray diffraction analysis to obtain their chemistry and mineralogy. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis was conducted to determine the microstructure and compositions of the ash deposits. The results showed that the fouling of the slag tubes was mainly attributed to Fe2O3. At the three-stage heat exchanger, complicated sulfates played an important role in the deposition formation. The deposition mechanism of sulfates gradually changed with the gas temperature at different stages of the heat exchanger. NH4Al(SO4)(2), Na3Fe(SO4)(3), and KAl(SO4)(2) were more prone to deposit on the surface of the heat exchanger than ammonium salts [e.g., (NH4)(3)H(SO4)(2)] at higher temperatures (>340 degrees C). (NH4)(3)H(SO4)(2) was found in the ash deposits at the outlet of the second and third stages of the heat exchanger, where the flue gas temperature was below 240 degrees C.