Determination of ash-forming elements directly from solid biofuels is essential in the prediction and prevention of ash-related problems like slagging, fouling, agglomeration, and corrosion in FB-boilers. In this contribution, we report the characterization of reed canary grass, straw, pine and spruce needles, as well as two peat samples by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and scanning electron microscope energy dispersive spectroscopy (SEM-EDS). For comparison, their ash-forming elements have been determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after digesting in a microwave oven or, in case of chlorine, after leaching with tetramethyl ammonium hydroxide. The analytical data are in agreement with those recorded previously for the ash of the biofuels in question and are consistent with their relative slagging, fouling, and agglomeration propensities. The prediction of the onset of ash-related problems is facilitated by use of fuel indicators that are conventionally based on bulk analytical data. The extension of the use of these indicators by considering the compositional distribution of discrete particles in the fuel by SEM-EDS and LA-ICP-MS provides more detailed understanding of the properties of different fuels and their mixtures upon combustion. In addition to laboratory tests, fuel and bed samples were received from a full-scale 315 MW CFB boiler during its normal operation using a mixture containing 60% of peat, 20% of woodchips and sawdust, and 20% of forest residue as fuel. While the operation of the boiler was unproblematic, occasional formation of coating layers on the bed particles has been reported. This behavior can be explained by the distribution of numerical values of selected fuel indicators.