With increased co-firing of coal and different biomasses in power plants, there is a growing need to study and characterize the produced, often heterogeneous ashes and to assess their utilization as secondary raw materials. In this work, we compare different methods for characterizing the ash compositions. These methods include scanning electron microscopy with element analysis (SEM-EDS), X-ray fluorescence (XRF), X-ray diffraction (XRD), and element analysis using inductively coupled plasma emission spectroscopy (ICP-OES) for ashes dissolved in aqueous solutions. The methods and the pretreatments required for each analysis were studied in detail for three ashes: 1) coal, 2) peat and wood, 3) wood residue, peat and dolomite addition. The limitations of each method will be discussed. Finally, the most feasible methods were utilized to characterize five more ashes. This work gives guidelines for selecting a rapid and reliable method for analyzing the suitability of the ash for a particular application.