To understand deposit formation in the flue gas cooler, which is used to recover the exhaust heat from a 300 MW coal-fired boiler in China, a mineralogical study was carried out. Several deposit samples on the surface of the flue gas cooler were collected. Then, the samples were examined by X-ray fluorescence (XRF), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Mineralogical analysis showed that the deposits could be divided into three layers. The high concentrations of O, Si, and Al in the outer layer were indicative of the formation of ash particles in the flue gas, and the high contents of Fe, O, Cl, and S in the inner layer were indicative of the formation of iron corrosion products, in comparison to the interlayer. In addition, the relatively higher contents of F and N in outer layer and interlayer were interpreted as the formation of ammonium fluoroborate (NH4BF4) and its intermediates [NH4F, HBF4, (NH4)(2)SiF6, and H3BO3], which were further proven by XRD and XPS analyses. The inner layer defined as the corrosion layer was caused by condensed acid on the surface of the flue gas cooler when the heating surface temperature was below the HF dew point. The formation of NH4BF4 was due to the enrichment of fluorine and boron in the coal as well as the escape of ammonia from selective catalytic reduction (SCR).