The agglomeration tendency of five Scandinavian forest-derived biomass fuels was studied using an advanced fuel analysis, i.e., a combination of chemical fractionation analysis, controlled bed defluidization tests, and SEM/EDX analysis of bed samples. It is shown that all five fuels have a tendency to form bed agglomerates when fired in a fluidized bed with silica sand as the bed material. The agglomeration appeared to proceed by formation of a sticky layer on bed particles gluing them together. The layers on the bed particles contained Si, Ca, and K, and, in some cases, P. The combination of advanced fuel analysis by SEM/EDX showed that the soluble fraction of Ca and K (i.e., leachable from the fuel with water and acetate) may be responsible for the formation of the layer. Silicon may mainly come from the bed particles.