Deposit formation is considered as an important problem challenging the improvement of coal-combustion performance. Deposits usually result from the thermal decomposition of coal-borne mineral matter followed by impact and adhesion along the hot gas pathway. Several ash deposit samples collected from the heat exchange tube in the Atmospheric Fluidized Bed Combustor (AFBC) at Western Kentucky University, were used in the present study. The SDT-MS and XRD techniques were employed to examine the sample composition and the mechanism responsible for the deposit formation. This work especially concentrated on the investigation of the chlorine containing species and the behavior of chlorine during the heat treatment. It was proved that the alkali or alkali earth chloride would vaporize significantly under the tested condition, and these compounds were likely responsible for the deposit Formation.