Four ash samples which were deposited on four different locations in the atmospheric fluidized bed combustor, FBC, system at the TVA Shawnee Steam Plant near Paducah, KY, were employed in the present study. The SDT/MS and XRD techniques were applied to identify the sample composition and the mechanism that form the deposits. The major compound in the deposits from convection pass inlet, superheater and multiclone inlet was CaSO4, and the predominant minor compound was CaO in the samples. Also, CaS was found in the deposits from superheater by XRD analysis. In the case of recycle feed pipes, Ca(OH)(2) and CaCO3 were the major components, and CaSO4 and CaO the predominant minor compound in the deposits. The combined SDT/MS with XRD results are useful techniques for identification of unknown ash samples.