Sulfate and chlorate are the key species contributing to the formation of ash deposits in a biomass furnace. In this paper, biomass combustion has been tested in a drop-tube furnace, and the ash deposits on a heating surface have been sampled. The physical and chemical characteristics of the deposits were analyzed using transmission electron microscopy in order to investigate the development of the ash deposits. The laboratory results have been compared with the ash deposit samples obtained from a biomass-fired power plant furnace. The results from both the laboratory and the plant samples indicate that most of the alkali chloride has been homogeneously converted into alkali sulfate in flue gas before deposition on the heating surface and that sulfate aerosols play a dominant role in the formation of biomass ash deposits. Finally, major pathways of alkali sulfation and the role of sulfate aerosols formed in the biomass-ash deposition are proposed.