This paper presents the results of standard fuel analyses of 112 different fuels, along with the results from chemical fractionation through selective leaching. The samples, obtained from boilers during the period 1995-2010, represent the following fuel classes: coal, peat, wood-derived fuels, agricultural wastes, and sewage sludge. The leaching results show that coal consists mainly of insoluble ash forming matter, dominated by different silicates. The ash-forming matter in wood-derived fuels is mainly soluble and is dominated by potassium, calcium, and phosphorus; the insoluble ash-forming matter originates from soil contamination. The ash-forming matter in peat has the characteristics of both wood-derived fuels and coal. In agricultural waste, the ash-forming matter is mainly soluble, even though it may contain some silicon, and is dominated by potassium, chlorine, and phosphorus. The ash-forming matter in sludge is mainly acid-soluble or insoluble. It is also important to note that the aluminum silicates present in sludge may capture volatile potassium during combustion, thereby reducing the risk of deposit and corrosion. The ash-forming matter in sludge is often dominated by phosphorus precipitating agents such as iron sulfate and aluminum sulfate. The study demonstrates the value of chemical fractionation through selective leaching as a tool for explaining ash behavior in FBC.