||In this study eight types of deposits were collected from the surface of water-wall tubes, in the lower and middle sections of three pulverized coal boilers. The chemical characterization of all deposits formed under reducing atmosphere was explored. Based on thermodynamic calculation of Pb-Zn-O-S system, the causing for Pb and Zn enrichment in deposits was analyzed, and then the deposition mechanism of Pb- or Zn-rich particles was clarified. The results show that, Fe, Zn and Pb sulfides are contained substantially in deposits and their enrichment has a sequence of time in the profile section of deposits. Pb element as PbS is enriched in the inner layer of deposits, due to the direct condensation of PbS(g) or and Pb(g). While the Zn enrichment in layered deposits falls behind Pb element. Zn element exists largely as the form of ZnS and ZnAl1.04S2.13 in the depositing layer, which is attributed to the heterogeneous condensation of Zn(g) rather than ZnS(g), the sulfidation reaction of Zn(g), and the subsequent thermophoresis deposition of fine particles. The mole ratio of S to Pb + Zn in deposits could be used for roughly and quickly evaluating the degree of sulfide corrosion.